ANOTHER BIG MERCHANT dumps American Express.

It’s fair to say that this has been a terrible month for American Express. It also lost a partnership with JetBlue and, even worse, an important court case. American Express cards are more expensive for merchants to take than other cards, so merchants would like to ask customers to use other cards — only they can’t, because American Express merchant agreements forbid this. They can refuse to take the cards in the first place, of course, but then they lose customers who don’t have a Visa or MasterCard.

American Express has taken a unique approach to a competitive field. Credit cards are what economists call a “two-sided market”: They need to get both customers to take their cards and merchants to accept them. Visa and MasterCard keep their fees relatively low in order to woo merchants; AmEx has kept the fees high and passed some of that money back to customers in the form of lower fees and higher rewards. Essentially, it gambled that merchants wouldn’t dare refuse its cards as long as enough customers preferred to use them.

That gamble looks to have been a bad bet. A federal judge just ruled that it cannot place those sort of restrictions on merchants, which means American Express will have to lower its fees or lose transactions as merchants ask customers to put that purchase on another card.

I try to use American Express for everything where I don’t pay cash. I don’t like being asked to use other cards. I have other cards, but I don’t like to use them.