As far as I know, Activision is behind that ban, and it was already in place with MW2. It's a shame really, because they wouldn't have to do that - the international version of MW2 is not banned in Germany, so it's not even like activating it on Steam was somehow against the law. (In the case of Black Ops, it can't possibly be as the game isn't even out yet)
Even if it was, there doesn't seem to be any problem: You can still activate an imported PEGI version of Left 4 Dead 2 (which is banned* in Germany, for whatever reason), so Valve themselves don't really seem to mind.
But I won't discount on Steam's role in this case: Mandatory Steam activation is the very reason a publisher can do this in the first place.
* Note that a ban like this only affects commercial distribution: It's still perfectly legal to own or play the game.