Of course the real problem isn't which back button behavior too choose, it's that there's a choice to be made at all.
Both back button behaviors are perfectly valid and convincing arguments can be made for each, so the choice is pretty much arbitrary. But if Google doesn't enforce any particular behavior and leaves it up to the developers, the only possible result is confusion.
Additionally one could make the argument that the whole back button concept itself is inherently confusing, precisely because the right choice of behavior isn't obvious.
Both back button behaviors are perfectly valid and convincing arguments can be made for each, so the choice is pretty much arbitrary. But if Google doesn't enforce any particular behavior and leaves it up to the developers, the only possible result is confusion.
Additionally one could make the argument that the whole back button concept itself is inherently confusing, precisely because the right choice of behavior isn't obvious.