The back button being placed at the top is because it's the clearest place to put an indication of a hierarchy. Remember, iOS has always made it explicit where

The back button being placed at the top is because it's the clearest place to put an indication of a hierarchy. Remember, iOS has always made it explicit where a back action will take you — the arrow button isn't just an arrow, it also contains text that tells you where it's going. It makes perfect sense to put such a control at the top of the screen next to the title. The interface is clear: you are here, you can go back where you came from — and these two things are grouped together. Android has now adopted this which is great, because it's still a crapshoot where the hardware back button will take you on an Android phone — back in the navigation hierarchy? Out of the app entirely and into a different one? Will it just dismiss a popover? In the past moreso than now, the button's behaviour could almost have been described as entirely arbitrary.