It is very complicated, but I don't think they have a leg to stand on. What I find anoying is that they are only doing this due to the "violance" in the game, and are making a big thing of that. I'd not heard of the game, left alone played it, but it sounds no worse than a lot of games I have played and not considered violant, afterall it's killing aliens, in a war situation, and the player very much seems to be the good guy.
Anyway, when the Church talks about violance, it's a bit pot calling the kettle black, afterall, the Bible is a very violant book, and a lot of real violance has been and still is down to religion. I wonder if they would object to a game where you played the army of god on judgement day and went around killing unbelievers.
Then again, I'm suprised they arn't also objecting to the fact that the game contains aliens, when the Bible makes not referance to other life in the universe.