It's not installing it on multiple PCs it's the problem - it's the fact that it's having to contact a separate 3rd party activation server to install it. This system has caused problems before with Riddick, with players being unable to activate the game AT ALL, and being given the runaround between Steam, Tages and Atari, all denying responsibility.
Besides, Steam games are supposed to be locked to an account and not a machine. Steam DRM is fine for a game like this, especially as it also has an account lock for multiplayer.
Here's hoping however that they patch it out after a couple of weeks like they did with Bad Company 2.