See comment #20 http://www.boingboing.net/2010/10/08/cbc-agreement-with-t.ht... for an explanation from the CBC.

tl;dr: Most music licensed under Creative Commons disallows commercial use. So, almost all authors are prohibiting the CBC from using their work.

Have there been any surveys done to see what CC licenses are in use?

I mean in any case it seems a little silly that one would ban all of them and not just ones marked "noncommercial."