Sure there is. Section 2 of the Sherman Act makes it illegal to "monopolize, or attempt to monopolize, or combine or conspire with any other person or persons,

Sure there is. Section 2 of the Sherman Act makes it illegal to "monopolize, or attempt to monopolize, or combine or conspire with any other person or persons, to monopolize any part of the trade or commerce among the several States, or with foreign nations." You may think that Apple's conduct does not fall under that provision, and it remains to be seen whether the federal courts will agree, but if the legal argument were totally baseless then Apple wouldn't have gone to the Supreme Court to shut it down at a preliminary stage.