Yeah, most companies that create animated shows and movies have a different copywrite law than say, an artist who does nothing but art and create OCs.
These characters are allowed to be used a references, and there are many reasons behind this. Many companies will develop a creature or person that you can identify as a very close image to another character from a show that was developed by a different company.
Also, when people make fan arts for these shows it helps give them publicity, so they use that as a form of advertisement. When it comes to simply posting an image from the show/movie, heavily referencing an image, or taking/slightly changing a fan artist's work this would fall under the category of theft. A lot of companies are considered publicly owned, so there's a lot of things they can't try to sue someone for.
Also, there are a lot of fan art images floating around. If they tried to stop everyone they wouldn't have time to things that could make them more money like developing more episodes. The only time I would consider it theft is if all someone sold on here was fan art, and was making a decent profit from it; since all they are doing is taking other people's ideas, and never coming up with their own.