Oh, it's definitely not new. Apple has been rejecting games and apps with Exit button for years. But I guess some games might have been lucky and came through
. I don't know any though.
On Android, you can pretty much do whatever you want unless you violate the terms of Google Play, since there is no review process. It's just considered a bad practice to interrupt the life-cycle of the app.
The general idea is, that when you go to chat for a while, browse some web, check your Facebook, or even go to sleep, you can always go back exactly where you left of. Since the app consumes no resources while suspended, it creates great user experience. The system's memory management decides by itself whether it needs the RAM previously addressed by the app, and only kills it if it needs to. Usually, games are so memory intensive, that they are the first to go anyway. There's no need for the user to be involved in the memory management process. It's actually pretty neat to come back exactly to the same spot, as it creates an illusion of window-based multitasking even though the app was actually sleeping the entire time. And for low-memory apps, this works very reliably. For games, not so well, but you might still enjoy the option to reply a message or read some walkthrough, and jump right back into the game.
This entire memory management thing applies to both platforms, and Apple likes to make sure that developers respect its intentions. Google likes that too, but doesn't enforce it. So, while it's possible to do on Android, we prefer to follow the best practice of the platform.