I think the official reason (I read somewhere in documentation) is because they don't want web sites to suck up mobile bandwidth by automatically streaming stuf

I think the official reason (I read somewhere in documentation) is because they don't want web sites to suck up mobile bandwidth by automatically streaming stuff. You can have videos autoplay, yes, but videos you can overlay with controls, whereas audio can be integrated into an interface in such a way that the user has no means to stop playback.