Perhaps Apple uses title case in alert titles because it is just a little harder to read quickly? The slightly awkward looking mess of caps breaks up the word shapes, and actually makes you pay attention to each one. It might improve the effectiveness of alerts like “Erase All Data?”, keeping us from automatically tapping “OK.”
And just look at the care it was written with. The UI copywriter capitalized On, because it is not just any old preposition, but an adverbial one, part of the key action phrase Turn On.
You won’t see that at Google, where engineers make the interface and can’t be bothered understand the nuts and bolts of English.