I don't see what's the problem here. They give you a chance to reclaim your own game by sending them a photocopy of the serial number along with the serial they send you. I did that for my Dawn of War 2. I mean they could have just said that the game is registered to another Steam account and you cannot retrieve it at all. Therefore you'd have to go out and buy another copy of it.
And like it or not, more and more games are going on Steam. You're not able to play them without Steam now, neither will you be able to play them without Steam in the future.