So what happened after the legislation was put into effect?

“Most large retailers have seen significant cost reductions as a result of the Durbin Amendment, yet to date there is no evidence that those cost savings have been passed through to consumers,” a study by the George Mason University School of Law said. Other academic research has come to the same conclusion.

It shouldn’t be a surprise. Somewhat similar rules aimed at lowering fees were put in place in Spain and Australia, and the outcome was the same. “In Spain and Australia, the regulation of interchange fees (I.F.s) resulted in a transfer of costs from retailers to consumers,” according to Europe Economics, a consulting firm. “Retailers’ costs fell as they paid lower merchant service charges (M.S.C.s), but this cost reduction was not passed on to consumers in the form of lower retail prices.”

Even Judge Garaufis seems to recognize the possibility that his ruling won’t have the intended effect. Buried in his thoughtful 150-page decision, he acknowledges that at the about three million retailers that don’t accept American Express, prices haven’t come down, nor have the stores tried to create competition among the card companies — for instance, by encouraging customers to use one card instead of another with higher costs, a practice called “steering.”

“Amex correctly notes there is little evidence of widespread steering at these merchant locations,” the judge wrote, “or that price competition among the networks has increased in the four years since Visa and MasterCard agreed to abandon their antisteering rules for the merchants operating these locations.” (Visa and MasterCard used to have similar nondiscrimination provisions but got rid of them as part of a different settlement with regulators in 2010.)

American Express, of course, took issue with the judge’s decision, saying it “would harm competition by further entrenching the two dominant payment networks, Visa and MasterCard. Only a small percentage of Visa and MasterCard holders carry American Express cards. By contrast, most American Express card members carry a competing card in their wallet,” the company said in a statement last week.

There are other, perhaps more basic, questions about the ruling.

First, it is worth noting that retailers aren’t required to accept American Express. Indeed, nearly three million retailers that accept Visa and MasterCard do not take American Express. When a retailer chooses to accept the card, it also accepts the terms of use, which outline that the merchant cannot engage in steering.

That makes sense. It would seem odd for a merchant to accept one brand of credit card and then encourage its customers to use a different one because the fees are cheaper — unless it was trying to piggyback on the brand of the more expensive card to get customers in the door in the first place.