I thought it would be easy to use Windows Product ID? I doubt it is easy to modify Windows Product ID. When you buy an app, it will just tell the store which Windows Product ID bought it, so, it is OS bound, which in term, is device bound. The case about you buying an app on someone else's computer is fine, it would use whatever Windows Product ID at the time of purchase. Because the idea is basically the same. If you ended up buying an app on that device which you don't own, it also means the device is quite cheap and placed everywhere for you to use. Thus, those devices should get some rewards. The app is still tie to your account, but, the profile at the moment of purchase is based on Windows Product ID.