pdifolco --
Copyright holder's option. For instance, there used to be an "Aliens" total conversion for Doom, but then Fox went after them. And since both Star Wars and Star Trek have what are probably /very/ lucrative, possibly exclusive licensing deals in the computer gaming arena (LucasArts does its own, while Simon & Schuster made a line of Star Trek games, if memory serves) turning games into unauthorized SW/ST games does indeed risk their wrath. This gets worse if actual copyrighted art is used, since unlike trademarks, copyright doesn't have to be defended vigorously at all times in order to be able to defend it later.