It's hard to construct an argument that they should, other than reaching for "common carrier" status or some kind of anti-trust. To the extent that they're acting as a "publisher" they can publish what they like.

But it's still strange that Microsoft is forced to put in browser choice (and in some jurisdictions media player choice, I think), while Apple get to be a platform-monopolist. The usual argument against this is the non-dominance of the iPhone; so long as Android is the most popular OS and allows some choice of stores, I don't think anyone's successfully going to make a case against Apple.