People pretty much never change the operating system of their ARM devices anyway. They are a lot more heterogenous thing than PCs and getting any OS to run on some specific ARM device is always quite big a task. There's of course some examples of this happening, say TouchPad or N9 Android ports, but these devices already ran Linux kernel.
But it doesn't have to be that way. Even with locked bootloaders on Android devices people are finding ways to install custom builds of Android. Why not custom builds of GNU/Linuix, FreeBSD or even Windows.
How many people do that to their PC? The vast minority. The situation that is on phones and tablets isn't all that different than what people do by choice.
The majority of people don't care. If the people don't care, why would a company do it?
Sorry, but I did it (the tablet I'm writing this on runs a custom OS). The only reason for Microsoft to forbid changing the OS is to limit competition and thus harm the consumer who will have less, inferior (because if you have no competition, it's no longer worth investing in improvements) choices.
