My store has a ban on earbuds which they cite as an OSHA safety thing; I don't know if it's an official OSHA rule, but I do know OSHA has general-purpose rules that back up an employer when they deem something is unsafe. Anyway, I can get behind that as I've had to scream at someone with a single earbud whose music was so loud they still couldn't hear anything. Also, a lot of people were making phone calls using their earbuds and the attached microphone, which distracted them even more than music.
Then again, some people can't push a cart and chew bubblegum at the same time - throw in another factor like music and all sorts of accidents start to happen.