This story is all over the science fiction and fantasy segments of the internet this morning, but just in case you’ve missed it, a few links:
The estimable John Scalzi weighs in on the topic, as do many commenters.
I particularly like the person who provided a link to a Wikipedia list of actual space marines. I have no idea how many, if any, cosmonauts were the then-USSR’s equivalent of marines, but I’ll bet that somebody on the internet does.
Over at Making Light, they’re on it as well.
And why did Games Workshop choose to go after a self-published author, and not, say, the Heinlein estate or any of the other fairly large gorillas who have used the term and the concept of “space marines” in their science fiction since the late nineteen-thirties at least? For the same reason that school administrators, when they decide to implement a “zero tolerance” drug policy, go after the honor student who gives her best friend a Canadian Tylenol for cramps, rather than the apprentice hoodlums selling coke and steroids to the football team out in the school parking lot — they can get the desired result (the appearance of vigorously carrying out policy) without the fear of serious repercussions (in the form of lawsuits or busted kneecaps.)
Dr. Doyle's Blog 
While I fully believe the attempt to claim Space Marine is flawed, there is a context to their actions that removes, for me, the scent of odium.
GW have been involved in a protracted legal dispute with miniature manufacturers who were selling replacement/add-on parts for GW’s tabletop game. As that situation arose from their not launching a salvo at the first sign something might be infringing it appears they have suffered a knee-jerk swing the other way.