There are a mind-boggling number of science fiction stories, and a much more limited set of common sci-fi tropes. The odds of multiple independently-originated stories sharing similar plot elements is high.

When someone as popular as Cameron publishes something new, the weirdos come out of the woodwork, smelling the scent of a deep pocket. Sometimes the rich folks even settle with the little noisy folks, even if they didn't actually plagiarize.

And then there's the enlightened ones who would rather not bother with lawyers. Larry Niven, for example, told me that he would not be getting on Microsoft's case about the first Halo game. "You can't copyright rings", he said.