In fact early rock'n'roll was in my humble opinion very much a form of folk music. It wasn't sung in the British Isles 200 years ago, but it arose from the grassroots level, went against the mainstream commercial music business, was certainly no guarantee of financial success, and was looked down upon by most people including/especially people who were playing and singing pure (read sanctimonious) folk music. And it's roots were in blues. I suppose that, along with it's relatively recent origin, the main reason that we don't consider it to be folk music is because of the technology of it's dissemination (recordings) which, along with copyrights and royalties, tended to preserve the originators of the song (the guy's names were on the record labels). But I remember the days when rock'n'roll was still ridiculed, even in MAD Magazine, and you never heard it in TV or movie soundtracks (let alone commercials). It wasn't until the late '60's/early '70's that people discovered you could make money off of it. That's when it ceased to be folk music.

Okay, Jon, take it easy big fella, get down offa that soapbox 'fore someone gets hurt.