It's bad UI if the developer programs bad UI. The back button returns you to the previous activity by default, you can override it but you should have a good reason. If I downloaded shovelware from the app store and a button labeled 'Play Game' actually took my photo and uploaded it to hotornot, I wouldn't blame iOS for allowing buttons to do things other than what they said.

That's also another thing: iOS is app-based, Android is activity based. You don't have the same navigation possibilities in iOS so you can get by without it. When you flow from one activity to another, you want to return to the previous activity rather than the previous app. Hence the back button.

I talked about UI indications for the previous app elsewhere, but it's dishonest to present the lack of indication as a point against Android when iOS doesn't even have a precise analogue of this, and what feature it does have already exists Android by long-pressing Home!