On Monday Apple will ask the Second Circuit Court of Appeals in New York to overturn an antitrust verdict. U.S. v. Apple boils down to this: Amazon possessed a 90% market share in e-book sales, and Apple developed a method of carving into it, increasing competition. Nevertheless, federal Judge Denise Cote held that Apple violated the Sherman Act.

Apple’s appeal is important to more than the company. The Second Circuit has the opportunity to consider the appropriate antitrust rules governing competition between marketing platforms—an...