Bullshit, this fallacy keeps going in every Steam thread and is just pure bullshit. They are not forced to anything. Steamworks provides a service to the developers, in terms of online implementation, userbase and DRM, a service that would cost lots of money and time if the developer had to do it. And the only "price" for that service is that it requires its platform to work. How is not that fair?
Others distributors can still sell the game with Steamworks and get their cut, but of course the cry fool because they know once people are exposed to Steam, they rarely go back. So what are they complaining about? That Steam is too awesome? Come on...
If they want to compete with Steam, they must offer what Valve does in terms of service, for ursers and for companies. Or find their own niche like GoG and Gamesrgate does. But complaining about a company for being simply better than them is pure stupidity.