Music might really be their main problem. Basically every video has some music in it. If not in the foreground, then in the background. Even game soundtracks in Let’s Plays are often under license. So the moment someone plays any such video, the content mafia comes around the corner with their baseball bats in hand collecting their tolls.

So I assume if they have to pay for music any way, they figured they might as well include a tailored music listening experience with Premium.