See, the problem here is that you're saying "But they should be able to make a game and base it on someone else's work and intellectual property and there shouldn't be anything anyone can do about it" - this isn't allowed. I'd argue that the owner of the rights might worry about people making bad work based on it, but considering that in practice bad games happen all the time based on movies and books, and bad movies happen all the time based on games, that doesn't seem to be a concern - it's more "you aren't paying us, so you aren't allowed to do this. Someone might choose to play your thing instead of one of the official games, or any future game(s) we make!"
Obviously some copyright owners don't bother stopping people from making their own games or mods based on their own works, and some even encourage it. There are a bunch of mods for games based on the A Song of Ice and Fire books, all unofficial, including a CKII mod, for instance. Recently there have been a couple official games, but they've both received generally bad reviews. The TV show has met more success but has not really been completely accurate, eventually diverging from what actually occurred in the books (besides that the ages are all wrong, it's filled with sex and nudity, etc):
Similarly JMS does not stop people from making games set in the Babylon 5 universe, and has given his encouragement for their creation in the past. I think his thinking is that fans are far more likely to obsess enough about it to make a faithful and good game (even if they take forever to do it), rather than a game studio which most of the time seem to do terrible jobs on adaptations unless they are really enthusiastic about them and have a lot of time and money. That's what I think, anyways, and I am for some reason assuming that his thinking is similar because of his policies on the matter and the way he has never licensed B5 out to any video game companies (there have been non-video game adaptations, e.g. an RPG I think, also a game with cards, neither of which is as expensive).