Could you tell us how the App Store exclusivity significantly enhance usability? Apple already controls the native API and the core apps, how can they need more? Do they really need to ban poor apps? Do they really need the censorship?
Usability can't explain all the control. There are other reasons.
The control is there, but it's far from the primary or even a major driver, I would claim.
The control and banning of poor apps reflects something not only Apple is doing - see Google and their "banning" of content farms and ad farms.
Junk breaks discoverability, and discoverability is one of the more fundamentally important notions that users have started to get in the last few years.
A few years ago you'd get panicked calls from users who are afraid to click on anything, because the slightest misstep means an app crash, an OS crash, or just plain data loss. Now we've finally developed software that's intuitive enough and appeals enough to people's natural assumptions about software, that this is no longer the case. People are happy to tap away and discover new features of their devices and software, and that's great.
By allowing crapware and malware into the system, we degrade user trust, and we go back to the days where users are apprehensive about clicking on anything, for fear of losing their data, for fear of getting a virus, etc etc ad infinitum.
tl;dr: The "control" angle, for the most part (except some clauses such as App Store exclusivity) does enhance usability.
> (except some clauses such as App Store exclusivity)
To me, this exception is the most part. I wouldn't complain if they had relinquished it.
As for viruses, I am beginning to think that an OS that crashes is just unacceptable. (Which may rule out even OpenBSD, but I'll bite that bullet. http://www.vpri.org/ gives me hope).
Usability can't explain all the control. There are other reasons.