So yeah, there's that one little catch!
Valve loves Linux. I think that's pretty clear now from their whole Steam Machine initiative. The dumbed down version (because I am dumb) is this: Valve wants to move away from Windows and sees Linux as a way to do this.
As a result of this Valve is super keen to interact with Linux developers. One way to do that is to offer every single goddamn Valve game on Steam — future, past and present — to Debian Linux developers. Makes sense since Steam Machines use Debian Linux as a base.
If you are an official Debian Linux developer you might want to head here for more information. The rest of us can just cry big fat salty tears into our empty wallets in despair.