That is not the legal definition of a monopoly. By law, a monoply is a company that is large enough to either dominate or significantly influence a market. Since there is a large market for iPhones and Apple exerts complete control, a strong argument can be made that they exert monopoly power in violation of the Sherman Antitrust Act.
I'm glad someone is finally making a case for this. I've long believed that Apple operates their store illegally.