the restriction guarantees a level of quality of service you cannot get in many other places.
Actually, I'm not so sure. The iTunes store, for example, doesn't, as a matter of course, allow you to re-download items you have bought, for example if your hard drive has crashed. They assume (fairly reasonable, but for those who use a Mac, 'cos it "just works", not always aware of things) that the user will make a backup on another device (perhaps the "Time Capsule" will be recommended, for additional Apple profits)...
One restriction, that of not having a microSD reader in their devices, must be a major pain. I don't buy music online, but will 'rip' tracks from CD into MP3 format. That means my cheap + cheerful player costing all of a tenner, can play such tracks as I have stored on it.
With mobile phones, however, I am easily able to simply plug in an 8 GB microSD card with music selection 'A' and another day, plug in selection 'B' - OK, it's a minor difference in one way, but major compared with having to buy a replacement device (at a possibly inflated price, comparing the price of storage bought in bulk, with what Apple sells the additional storage for).
I use iTunes, but mostly to download podcasts, and if I was buying a device such as an iPhone, I think I'd be dumbfounded if I had to have some computer (with iTunes) to plug into, before I could start using it. (I'm going on history, here, and don't know if that's still true, but a major disincentive in my view... Why should I need a computer if I'm just buying a mobile phone ?
I'm not going to try to defend Apple from some who attack them - this is being written on an (old) iMac, after all, and the demo version of 'DEFCON' runs on it, so I'm happy enough (though DEFON does not run on 3 other iMacs I own, and there's a fair bit of software that won't run on older Mac kit)