You are allowed to buy most of these games in Germany. The problem is that you cannot advertise them in any way or present them on the shelves, so most publishers either don't bother releasing them, because they would sell like crap or the developers make massive changes to the game.
There are only few notable games that are truly banned in Germany. Mostly those that either glorify National Socialism (older PC games) and those that glorify violence (still present).
Those are banned games:
http://www.schnittberichte.com/svds.php?Page=Beschlagnahmen&Kat=Spiele
http://www.schnittberichte.com/svds.php?Page=Indizierungen&Kat=Games (everthing with a "B" before the title)
There are lots of older games missing in their lists though.







