Let's assume millions did rush to buy it and install Linux.
Think of how many of them would be installing it just to play one game. Most would gripe about not being able to do something they're used to in their other OS [0].
While I love Linux, there are some pain points on desktop. Just getting it installed on your machine is not always trivial even when you know what you're doing. Something you've done a thousand times sometimes just doesn't work for no real reason and you just have to try again [1].
Linus help them if they need a proprietary driver [2].
I imagine most of the people trying Linux for the first time would be trying it with the intention of playing a game instead of learning a new system. That sets them up to be annoyed every step of the way and walk away angry at the new system (Linux).
On top of all that, it could cause some frustrations for people like myself who try to help others install and learn Linux. I imagine having several conversations with friends/family thinking they want to finally learn some of this Linux thing I keep talking about. Only to discover it's just because they want to play HL3.
[0]: http://linux.oneandoneis2.org/LNW.htm
[1]: I realize this happens with computers, in general. The point here isn't to bash Linux (heh).
[2]: obviously, not a fault of the Linux ecosystem, but that doesn't change the fact that it is incredibly frustrating.
EDIT: formatting.