As far as I can tell, it's a combination of the licensing being different (as in the terms of your license probably only cover the United States to start), and the CRTC's content laws and other regulations being annoying enough not to bother with considering our relatively smaller market size.
(I think it's considerably easier to go from Canada to the US than in the opposite direction.)
Looking into it, apparently the CRTC believes that internet audio and video broadcasting services are under its jurisdiction, but it has exempted all such services from regulation. The notice is at http://www.crtc.gc.ca/eng/archive/1999/PB99-197.htm.
