They don't own the rights. (And neither does Dan O'Bannion...) Sega have licenced (i.e. rented) the games rights from 20th Century Fox, giving them right to produce games, under approval from the movie company, for the period of the licence. Aliens versus Predator, which Rebellion are making for Sega, is one of several Aliens games currently under development, but will be the first to be released.
I also worked on a Star Wars game for Rebellion, but that doesn't mean they own the rights to that either.