I'm surprised nobody's pointed out that the mediaeval origin of 'ballad' is in dance- "balar entre nos" - and suggests that the words were originally sung (perhaps improvised) while dancing. I don't know how that affected the folkish usage of the word, or for that matter the pop usage, but it does suggest that the word's slept around a bit.

Incidentally, while checking the spelling of 'balar' I ran across this very interesting blog about language, culture, children's songs, folklore and much more.