The decision was overshadowed by other blockbuster cases and the announcement of Justice Anthony Kennedy’s retirement, but the Supreme Court last week delivered the most significant antitrust opinion by the Court in more than a decade — one that made it extraordinarily more difficult for the government to rein in certain companies that abuse their market power.

The case was Ohio v. American Express, and it arrived against a backdrop of growing public recognition of the excessive clout wielded by corporations over American workers and consumers, and rising interest in anti-monopoly law and policy, especially on the left.

In it, the Court dealt a huge blow to the ability of government and private plaintiffs to enforce existing antitrust laws, making it easier for dominant firms — especially those in the tech sector — to abuse their market power with impunity.

How American Express exerts pressure on merchants

This case asked whether certain restrictions American Express places on merchants violated the Sherman Act, which prohibits certain monopolistic behavior. American Express, like other credit card companies, provides services both to merchants and to cardholders; it “mediates” transactions between them.

For decades, American Express has contractually forbidden merchants from encouraging cardholders to use its competitors’’ cards. So even though the fees charged to merchants by American Express are higher than those charged by Discover Card, for example, AmEx banned merchants from advertising that fact or doing anything that could steer customers toward Discover. The parts of the contract that impose these restrictions are therefore known as “anti-steering” provisions.

In 2010 the US government and 17 state attorneys general sued AmEx along with MasterCard and Visa, which also had these provisions; the latter two agreed to settle and dropped the anti-steering language from their contracts.

After a lengthy trial, the District Court of the Eastern District of New York held that these anti-steering provisions were illegal. Specifically, the judge found that the provisions made it possible for AmEx to routinely hike merchant fees — 20 times in five years — without losing significant business, and that the provisions effectively blocked entry by other firms.

That’s because efforts by rivals to offer merchants low-cost alternatives couldn’t gain traction unless merchants could actually signal to customers that that AmEx was more expensive for merchants to use than other cards.

Because the provisions eliminated the benefits a credit card company could derive from a low-price business model — most obviously, a greater share of the market — all four major credit card networks charged higher fees than they would have absent the provisions. All told, the four major credit card networks collect more than $80 billion per year from merchant fees, which merchants then passed on to consumers in the form of higher prices for goods.

Critically, although American Express members reaped card rewards as a result of these higher fees — benefits like gift cards and flight upgrades — the judge found that these perks only partially offset the higher prices. (Of course, the low-income populations that don’t use credit cards end up paying the higher prices driven by merchant fees without any of the rewards, which means that AmEx’s anti-competitive behavior also enabled a wealth transfer from poor to rich.)

The Second Circuit Appeals Court reversed the district court’s decision. It reasoned that, since American Express serves both merchants and cardholders, plaintiffs alleging anticompetitive harm to merchants must also show that cardholders were worse off overall. They concluded that, while the government showed that merchants had been hurt, they failed to prove net harm to cardholders.

The Court’s holding represents a stark departure from existing law

Antitrust laws have never permitted monopolistic firms to wield their market power against one set of customers so long as they benefit another set of players. Yet this kind of “balancing” is exactly what the Second Circuit ratified. Consider: Under the logic the appeals court used, an anticompetitive scheme by Uber to suppress driver income would not be considered illegal unless those bringing the suit showed that riders were also harmed.

What’s more, the court said, plaintiffs have to meet this new burden at the very earliest stage of litigation.

Last Monday, a 5-4 majority on the Supreme Court upheld that approach. Not only does the decision show stunning disregard for core elements of antitrust law, it carelessly mangles long-accepted legal rules along the way to establishing its position. Perhaps most strikingly, it overrides or ignores facts established by the district court.

For example, the Supreme Court states that AmEx’s increased merchant fees reflect “increases in the value of its services,” even though the lower court expressly found that AmEx’s price hikes exceeded the value of the cardholder rewards.

In practice, the Court has shielded from effective antitrust scrutiny a huge swath of firms that provide services on more than one side of a transaction — and, in today’s digital economy, there are many (as Justice Stephen Breyer noted in a dissent he read from the bench to emphasize his concerns).

Worse yet, the Court left unclear what kinds of businesses actually qualify for this new rule. As the Open Markets Institute, for which I work, explained in an amicus brief, deciding an antitrust case using the amorphous concept of a “two-sided” market will incentivize all sorts of companies to seek protection under this bad new theory.

What kinds of companies might have more freedom to exert pressure on customers, as a result of this decision? Not newspapers, the Court said: Readers are “largely indifferent” to the number of advertisements on newspaper pages, even though advertisers are looking to reach readers. So someone suing a newspaper on antitrust grounds (say, for prohibiting advertisers from doing business with other newspapers) would not have to prove that a newspaper’s conduct harmed both readers and advertisers.

On the surface, the Court’s language suggests that the special rule would apply to Amazon’s marketplace for third-party merchants, to eBay, and to Uber — but not to Google search or Facebook. Indeed, the Justice Department’s antitrust division chief, Makan Delrahim, has also come to this conclusion about the scope of the decision. But the Court’s opinion hardly delivers a clear and workable standard for judges to go by.

One can imagine the reams of studies Google would commission to show that targeting users with advertising did indeed amount to a “transaction” with users that users highly valued — a showing that, if successful, would likely qualify it for the shield of the special rule. If so, Google might be able to impose exclusionary contracts on advertisers and significantly boost the prices it charges them. Amazon, meanwhile, can continue to squeeze the suppliers and retailers reliant on its platform with little worry about being charged with the abuse of monopsony power.

Federal judges generally lack the expertise needed to independently assess the hyper-complex economic studies that this new rule will spur. Rather than focusing on the conduct between a company and one set of its customers, the new rule requires a much more involved showing.

But the degree to which current antitrust enforcement turns on the competing analyses of specialist antitrust economists is something to move away from rather than expand. It’s misguided to expose yet another critical inquiry in antitrust to the vagaries of how any single judge happens to read the complex economic analysis at hand.

With the courts undermining antitrust law, it’s time for Congress to assert itself

The Court’s American Express decision comes at a moment when politicians, journalists, and members of the public increasingly recognize that America has a major market power problem and that we must revitalize our antitrust tradition. When companies have too much market power, they can depress wages and salaries, raise prices, block entrepreneurship, stunt investment, and exert undue political power.

For the Court to weaken antitrust further now is an effective way to draw the attention of members of Congress who are concerned about the growing concentration of corporate power. This includes US Sens. Cory Booker (D-NJ), Amy Klobuchar (D-MN), and Elizabeth Warren (D-MA) and Reps. David Cicilline (D-RI), Keith Ellison (D-MN), and Seth Moulton (D-MA) — all of whom have introduced anti-monopoly legislation in the past year. If Democrats should win the House or Senate, anti-monopoly legislation could, and should, be a top priority.

For decades our courts have constructed an antitrust regime at odds with the values that Congress articulated when passing the antitrust laws. American Express marks a continuation of that abnegation. While the judiciary has claimed for itself significant authority over shaping the substantive content of antitrust policy, it’s time for both the antitrust agencies and lawmakers to reassert their power. The time for a robust and muscular antitrust regime is now.