It's their platform, they can do what they want with it. Nobody's telling Microsoft they're "anti-competitive" because they have a lock on the Xbox marketplace, nor Sony and their exclusive relationship with PSN.
It's anti-competitive when you're talking about an essential service. The case against Microsoft was about Windows since at the time 95% of all computers used some form of Windows and they had a virtual lock on the market. Generally you need to be in a monopoly position to be considered anti-competitive. See also: Standard Oil, IBM, AT&T.
There is only one fundamental requirement: that the company's actions be anti-competitive. It is not necessary for the company to have a monopoly or even a majority share of a market (though these are factors). Furthermore, ownership of the platform is a significant antitrust factor because it drastically ups the risk and concern for anticompetitive behavior. (The Disneyworld analogy brought up below does not apply. Not only does it mix up antitrust law with property law, it ignores the crucial distinction between Apple and Disney: Apple openly invites others to participate in commercial activity on their platform, whereas Disney does not.)
Standard Oil, AT&T, and Microsoft may be the marquee cases, but they're not representative cases of the extent of antitrust law.
There are all sorts of gray areas here that Apple will have to get called out on.
Third party payments are a great example. Say I sell an app that lets you pay for movie rentals at a RedBox kiosk that does so by letting you select and purchase on your phone/tablet and sending the right instructions to the RedBox servers so that your rental is prepaid when you arrive at the box.
Do I owe 30% of what was paid to RedBox even if I as the app developer am not RedBox?
Then let's talk declining balance accounts, a subcategory of the above. For example, your lunch money account at college. Your mom might put $500 on that account at the beginning of the school year from her home computer. You then use your phone in the food areas on campus to pay from your lunch account.
Is this considered a virtual currency and therefore Apple gets 30% every time you pay for lunch? It definitely is based on how I read Apple's agreement.