It's very possible to pirate Steam games, the means are out there (emulated Steam authentication and the like). Steam is not the panacea for piracy a lot of people seem to think it is. It makes it a little harder, yes, and might make it less worthwhile for some people to do so. But it isn't a cure. Frankly, I don't believe piracy is THAT large an issue, it's just the latest excuse for developers and publishers to put out there when they make boneheaded mistakes. It's a scapegoat. Bitching about it here, though, is a waste of time, the choir already believes the preacher.
So, F them if they don't want to release a game on PC. It's their right to not do so, but I just wish they'd try being honest for a change, I get tired of being lied to by corporations.