Plaintiffs are several retail businesses who accept American Express cards for their customers’ purchases. Plaintiffs each entered into an agreement with American Express that contains, among other things, a class-arbitration waiver. Plaintiffs filed a class action complaint alleging that American Express’s “Honor All Cards” policy, which requires those merchants who accept American Express charge cards to accept American Express credit cards as well, constitutes an unlawful tying arrangement under § 1 of the Sherman Act. The named plaintiffs purported to bring suit on behalf of “all merchants that have accepted American Express charge cards.”