Having said that, I'm not too keen on the theory of copyrighting generic words, names and Ideas, capitals or no. Especially in this case, copyrighting "Space Marine" seems a bit like copyrighting your father's name and stopping him from using it in public.
To be fair, if you just call the guys in your story Space Marines as the actual name of their legion, then you should be punished for being stupidly unoriginal. So I guess they're actually doing us a favour by encouraging originality... by taking ownership of an unoriginal they used... *head explodes*.
Using the term "Space Marines" to refer to infantry who fight primarily in space isn't very creative, but suing someone for doing it because you did it first is complete and utter bullshit. Doesn't GW make enough cash screwing over their customers?