Space Marine is such a generic term and one that would have likely been independently created many times. Militaries around the world have Marines or the equivalent thereof and extrapolating the future would suggest we would have them in space.
Is their problem with calling them "Space Marines" or the description and depiction of them. If its just vaguaries what is to stop Superman owners from going after every series that implement the equivalent super hero. what about the Tolkien estate going after any one that has a high fantasy series dealing with elves or a magical ring.
I can see a problem if it was using very specific elements of the lore.