These games are purchasable in every store but steam, because the USK had trouble granting them a normal 18+ rating. Why? Because some politician a-holes can't see how such a game 'only' get's a rating of 18+. So they demanded that these games should be checked another time in the hope they get banned. Dead Space 2 had to get checked 6 times! by the USK which took 6 month till the politicians dropped it and accepted the 18+ rating. So DS 2 was available in EU for 6 month till Germany got it's final rating of 18+. And Steam just don't want to updated it.
Also another major point for buying games outside of Germany is cutting. DS 2 was lucky as only the Multiplayer was cutted from friendly fire mode which still results in German players turning FF off for EVERY other person on the server...