They retain the copyrights to the molds (I asked the MLP reps at Comic-Con and they were very definitive on this) and the artwork (they have been licensing it on everything from t-shirts to waste baskets lately. Maybe the problem is that companies OTHER than Hasbro put out the videos (Kid Rhino, maybe other companies overseas) and those companies don't have the rights to MLP art. Although . . . you would think they'd run into the same problem with G2 art. Hmmm. Or perhaps Kid Rhino first started planning the release of the MLP shows while Hasbro was still selling the G2 ponies, and therefore Hasbro just sent over a packet of G2 art when Kid Rhino asked for production art.
Anyway, technically Hasbro could probably come down on anyone using their art on a website. However, there wouldn't be much point in purposely pissing off fans of the line who buy the toys, so I doubt they ever would actually do anything. (Besides, they would send a cease and desist letter before actually suing anyone.)
Also, I think original fan art is safe all around.
~LM~