Why?
Say I own a shopping mall. I decide that I don't want a porn store operating there, so I refuse to rent to them. They go open up shop somewhere else. Everyone is satisfied (although the porn store owners might wish they had a slightly more visible location).
That's the Android world now. In the iOS world now, if Steve doesn't want porn in his mall, they get cut off at the knees because no third-party distribution is allowed. Period.
At the risk of repeating myself:
Google allows third-party app distributors. Google allows third parties to freely use their OS under generous licensing terms, and allows them to use the Android branding and Google app store if they play by Google's rules.
Apple does not allow third-party application distribution or hardware. Period.
Google does a lot of things with Android that upset me (giving their preferred partners early access to new revisions, for starters) but I'm completely cool with what they're doing here.
Sorry for the rant. :-)