EDIT (more): I can also imagine many situations in which I want to disallow a back button (like in my own application for example), or in which the back button could be expected to perform, but actually doesn't. Say I'm in a game and have navigated to a level, start the level, but then decide I want to exit. It isn't clear that the programmer has implemented the back function, or what level of destruction the back button will perform. I'm going to have to test the back button every time I press it to see what it even does, or what the implementation is. This is poor UI. In iOS, the developer is free to label the back button or to disable it entirely to remove ambiguity. I would say that this is far more powerful than occasionally switching to the previous app if I haven't navigated at all yet in the stack of the new app.