It makes sense that to play a game you bought on steam you have to have steam running and be on the same account you bought the game with. I don't see that as DRM any more than having to log into this forum with your account to post.
Also from what I can tell you get your activations back if you uninstall the game. 5 simultaneous installations. Since when will you have the game installed on five separate PCs at once? :P