Providing an OS feature only to first-party programs is a plainly anticompetitive practice. Using your privileged position in one market (cell phones/cell phone operating systems) to gain an advantage in another market (smart phone applications) that you withhold from your competitors is a textbook case.
I wanted to be outraged at apple, but I really can't. Read WinAPI documentation and try to count all "reserved" parameters for example. OS developers build features just for internal use all the time. Granted, this is just UI tweak so I'm not convinced it has to be private, but they probably just don't want to have to maintain that forever.
But this is exactly why you SHOULD be outraged.