The App Store mode is not "flat". Like most things presented as a pure meritocracy, the App Store is not a level playing field. If a headline app-maker wanted to do something Apple doesn't normally allow, then they can talk directly to Apple and get an exemption, and they usually do. (Imagine what would happen if a major user-magnet like Snapchat wanted to do something not in the TOS... you think Apple would pull Snapchat off iOS? Really?)
Doing this is in Apple's interest, if not in the interests of its other developers. iPhone sales are primarily built on the assurance that whatever app you're looking for will be in the App Store. Right now, buying an iPhone is a cast-iron guarantee of never missing out on the next big mobile service -- everything launches on iOS, and while most new offerings launch on both iOS and Android, some leave Android to their "expansion" phase. (I'm not talking about utilities or games, but the "new" service apps that are relentlessly advertised on mass-media outlets).
If a big-name property was suddenly made unavailable on iOS, it would send a big signal to customers and potential customers that, actually, iOS owners can be left out of the loop too. That puts iOS on the same level as Android to the vast majority of customers, and that's a dangerous position to be in when your devices cost so much more.