I don't disagree with that in theory, but like you pointed out... Who's going to enforce it in practice? Shouldn't have to be me, or anyone else who doesn't work for public transit.

Obviously there's a point where I'd say something if a person was being truly inappropriate, but I'm not risking my neck over music on a bus.

How is this not enforceable ? When someone is way to loud, drunk, promesticutus or drunk in the public space, you can call the police. Of course, there is appropiate mesures so police won’t come for music that is merelly loud but appropriate public behaviour is indeed enforced.