There is more than one form of monopoly... horizontal (a single resource controlled by one entity) and vertical (a single entity renders significant control through an entire vertical.
Now, it can be argued that Apple has enough of a vertical stranglehold that anti-trust rules can apply. It doesn't have to be an absolute monopoly to violate anti-trust law.
Yeah, people get overly hung up on the monopoly term. Back when antitrust was formed, it was aimed at trust busting. Meaning that several big names of a market would come together and decide how the market would operate, rather than compete.
And i do believe Apple has gotten a slap or two on the wrist for abusing their hold on content distribution. Ebook pricing for instance.
Now, it can be argued that Apple has enough of a vertical stranglehold that anti-trust rules can apply. It doesn't have to be an absolute monopoly to violate anti-trust law.