Because it auto-played when you didn't want to.

If you had Spotify running and then pressed the quick-play on your phones it would continue where it was, but after a reboot the iPhone would auto-play from Apple Music instead if Spotify hadn't been started.

So tapping play on your headphones would start those damn U2 songs "by accident" because it's the only thing that was on the Apple Music accounts we aren't using.. yeah no thanks.