App store may be theirs, but this is like after you install some third party e-mail client on your phone, Apple trying to force their way into telling you what e-mails you can receive, or who you can communicate with. (by proxy) Surely that's not reasonable. What does ownership of the phone mean then?
They could. They have the power to. Reasonable doesn't enter into it. That's why some of us object to their control over access to software for all their customers. I personally will not buy Apple products for this reason.
Everyone else's iphone isn't theirs though. Apple's abuse of their users is directly contributing to the consolidation of power in the world, which will likely lead to violence against and abuse of poor people.