Hmm, maybe this is what music is intended to do, but I don't think of it that way. When I listen to music while I run it's...background noise? I guess that's how I could best describe it. It puts on a certain ambiance that I find nice, but the focus is still on the running; effort, form, cadence, etc.
Might I sometimes be paying a little more then zero attention to music than to the running? It is possible. But I'm not sure, when I run without it my focus seems pretty similar, with my attention staying primarily on the same things.
It's called a thought experiment. I'm running around distracted all the time. Am I at fault the way runners with headphones are at fault? Am I a danger to others? If not, what is it that makes headphones any different?
Do you drive with headphones? Do you text while driving?
No, but a significant number of people listen to music on the Radio/CD/IPod/etc. while driving, so the effective answer is "yes, it is not uncommon at all".