Bell has been throttling "its own Sympatico retail customers [since] October 2007, and extended the practice in March to" smaller internet service providers. Bell is both a wholesaler, retailer and competitor. For instance it sells telephone services for which Skype is a direct competitor.
But many ISPs throttle their customers - did you know that Rogers and Shaw do too? This is mainly aimed at peer to peer file sharing which while it is famous for illegal file sharing is also widely used for perfectly legal file sharing
Regulators in the United States have taken decisive action against throttling by service providers. The Federal Communications Commission in August ordered Comcast Corp., the country's largest cable company, to cease its throttling of peer-to-peer applications.
UPDATE Bell won - but the CRTC is going to look at the whole issue of throttling next year