Oh .. weird. I remember it being taken down, but never checked to see if it was put up again, which it was. Guess RSA was removed earlier than I knew :<
Something wrong with blocking the sale of a game that is steamworks. Btw there has been no Space Marine DLC, so no idea if they block DLC as well. I would suggest you try and e-mail someone at THQ to get a definitive answer.
If not, is it possible to gift DLC through steam? Since that might be an option (pick the game up from Amazon and have Gaffers gift you the DLC)