You know this same claim was brought up by Activision against Valve, who in turn said 'What the hell are you talking about?"
Valve does not have a monopoly, a monopoly would be Valve making all the profits and desginers being FORCED to work with Steam, no one forces a developer to do so, and though I am not sure (maybe Larian can confirm or deny) but Steam allows you to use a distributor and get a much larger profit back as Steam wouldn't charge as much as say EA would per copy to distribute your product.
Exactly. Setch is dead on. Any claim of Valve to have any monopoly, in any form, is laughable at best.