It doesn't matter if it's something you enable explicitly, you're thinking about this the wrong way. Even if you enable it explicitly you have now compromised the uniformity of the utility of those buttons. There is now a cognitive cost of working out what that button is going to do every time you press it that didn't exist before, your quality of experience suffers. It's saving you from yourself.

Now you probably think that sounds anal retentive and patronising in equal measure but that's how you make good UX across a platform - by understanding the power of consistency to reduce the cognitive load of an interface.

Don't like it? You're welcome to go buy some random Android device with a random amount of buttons and widgets and d-pads and track balls that take a photos in this app and ring your grandmother when shaken to the left in the other app.

Personally however I'm glad this stuff is enforced. Sure, if it wasn't I could avoid apps that broke HIG, market forces and all that crap but that gets hard. I then have to try and find alternatives or put up with not having access to some functionality.

why does that only apply to physical buttons? The thing about the iPhone (and Android for the record) is that there are rather few interface commonalities between apps. It seems that everyone is designing their own widgets, styles, and creating vastly different affordances. The apps are all very flashy, but user interaction can vary pretty significantly from app-to-app.

I don't think that's necessarily a terrible thing. But why is that ok for the software (which is the bulk of the interface) and not the hardware?