Apple seems to prevent a company from having a line of text in their app that says: oh, by the way, you can purchase this texture pack or that asset on our website. I think it's the key to the problem. It seems difficult for Apple to argue that it is not anti-competitive.
That depends on the definition of the market. Big Co.s under antitrust scrutiny like to present themselves as being just small parts of some huge markets, like Jeff Bezos recently saying that Amazon is actually not that big when you compare it to _all_ commerce in the US. But that doesn't make it less incredibly huge in e-commerce.
Apple with iOS is in a much clearer situation though. Simply put, they have an _absolute_ monopoly on app distribution on iOS, and iOS is part of a powerful duopoly. Americans still can choose buying outside of Amazon (though maybe less conveniently), half phone-owning Americans can't choose anything else than the App Store.
