How is this different from a legacy bookstore? Apple is way more permissive than they are. You can't just walk into a legacy bookstore with a manuscript and say, "Here, sell this."

Major distribution has always meant going through filters. The publishing houses reject most of the stuff that comes through their doors, and bookstores only buy a fraction of the stuff that the publishing houses put out. If you are scandalized that Apple rejects one book out of millions, I can only suppose it's because you aren't familiar with legacy publishing. Traditionally, only a tiny fraction of books ever had a chance to be seen by readers.

(I say this not to defend Apple, per se, but to illustrate how good writers have really got it now if this is considered a subject of outrage. Just ten years ago, getting rejected was the expected outcome of trying to get a book published!)