I think back button suddenly closing the app is more hostile. When you do it accidentally, the recovery steps are painful (app switch, find app, focus to it etc). I totally understand why Netflix would prevent it from happening. I think iPhone solved it better than Android with the "super back" link or the app hopping gesture. Android could improve the back button with a long-press requirement in order to navigate away from the app too. In this current situation, Netflix is completely in the right to disable the behavior.