A game has to be rated by the "USK" in Germany who give away legally binding labels (over 0/6/12/16/18 years). An "18" rating would have some restrictions, but the games can still be displayed and sold.
When the USK thinks the game is harmful they give no label. The game can then be checked by the "BPjM" on request and could get "indiziert" which means indexed, put on the "Index" which is the list of harmful media. A version with an USK label can not get put on the index afterwards anymore.
Now, all games without a label have to be treated as if banned. I'll use banned instead of indexed from now on, but it's more a "banned from display". This includes all unchecked games, which of course means all import versions (Yes, even the english version of "Barbies Whatever").
Banned games can still be sold to adults, but they must not be displayed, no ads, no giving to minors. Imports and ownership are also allowed!
If a game is banned, this means less sales which is why the publisher censors the game and gives it to the USK for testing again. Even the "18" rating is feared to give less sales and a game might be further cut to get a "16" or lower. A fear of public opinion might also be a reason to not even release officially in Germany, as Microsoft does (Dead Rising, Gears of War) to keep a clean image for their Xbox.
After all the explanation:
Who buys in Germany gets a cut version, who doesn't want that can buy an import.
But since a few months import versions of Cod 6 can not be activated on a steam account in Germany any longer. Now it will be the same for Cod 7, this time from the beginning.
There still is no problem to activate import versions of other games in steam. That's why I think it's Activision that have their heads up their backsides again, with Valve as an enforcer with the only platform to play the game on. Steam has no way of age verification and so we got some patronizing moves in the past. Most games are "not available in your country" when there is no cut version, but for example the banned Hitman - Codename 47 and Prototype land in my shopping cart without a problem, the later being published by Activision.
I have no real interest in Cod or any of Activisions games really, but this behaviour sucks and shows where we'll go with closed platforms that can enforce this.
Most probably it has nothing to do with fear of legal problems or even consideration of adequate media usage of minors (ha!), it's just a way to sell the overpriced german versions and get rid of cheap imports.