It's a tricky subject, In Theory, a company could drop its law hammer and crush fan merch if they so desired, but it's generally allowed, since fan merch is small enough to not be a worry, and it doubles as free marketing, and attacking your fans like that is pretty bad PR. A big example of it are artist alleys in anime/game conventions, most content sold is fan merch and a company could easily shut down anyone selling their brand, but that's be a pretty stupid thing to do. There are some companies that are very strict about it so it's best to research, one I can think of right now is Sanrio, those guys go down hard on you if you're seen selling stuff from their IPs. Also comes to mind small indie companies that would actually have to compete on the same level as fan merch.