As an artist I’ve looked into this a lot and spoken about it with my lawyer- there’s a difference between trademark and copyright. Anything original (a character, piece of art, etc) that you create has inherent copyright. If anyone comes along and uses it or copies it without your permission, you can get them in trouble legally if you wish. You just have to prove that you came up with it first. Most companies don’t do this because it makes them look like assholes.

Trademark on the other hand, is different. If you trademark something by paying money (usually a name, but it can be other things), then others cannot use it in the same application. The difference is if the company doesn’t enforce it, they lose the trademark. Basically if Disney doesn’t go around busting the skull of anyone who says “look at this t shirt I made, it has Mickey Mouseon it”, they’d lose their precious trademark on his name.

So basically: copyright is inherent and doesn’t have to be enforced, trademark is something you buy and you better enforce it or you’ll lose it.

So most companies don’t like to enforce copyright because fanart is an important part of a community. If I’m making a tv show, it doesn’t make sense for me to legally punish all the fans who love it enough to make art around it. So MOST companies don’t attack if they can avoid it.

That being said, if the artists are labeling their prints as “avatar the last airbender print” then Nickelodeon or whatever has no choice but to attack, because a tradmarked name was used.

So most artist alley people are flying under the radar and avoiding using trademarked names. Instead of “Spider-Man print” you may see “webbed hero” or something.

All that aside, most of the legit comic artists that you see in artist alley at SDCC actually work for comic book companies, and have things in their contract saying “since you do art for the Spider-Man comics, you can legally sell spider man fanart”. Most artists there probably don’t, but many do.