As I understood from the small details written on their site, you are not obliged to link your game to the Steam network. The Steamworks package supposedly only helps you, as a developer, publish the game, it is like a tool. And why they are giving it out for free - well:
1) Valve rocks! ^^
2) The more people use their software, the more recognition they get and more people are getting used to using Steam, so more sales for Valve's games. If everything goes as planned for these guys, I believe Steam might become one of the top methods of publishing your games, and from a marketing point of view, that is always good!