Imagine if Microsoft did this on PCs. a) prohibiting the installation of non-windows store software (sideloading) and b) insisting that all purchases done via apps give them a 30% cut. I think this is a ridiculous practice on the behalf of Apple.
Easy, reliably DoS -- and the user has no means of fixing this vulnerability, other than rooting the phone and hacking around. Which is made ever less feasible.