Silly, but consistent. On other iOS devices, you cannot release apps that cover the functionality of standard apps provided by Apple.

I suspect it's also part of the design methodology. IIRC on the watch, the 'home screen' is the clock face so you should always go back to it when you press the digital crown(?). if you're using an app with a clock face, obviously that isn't going to work any more. When you press the crown you're going to go back to the inbuilt watch face, not the watch face in the app.