Developers who release their DLC via in-game menus make 100% of the profit of those DLC Sales, and Valve obviously makes nothing.
Valve feel that this isn't right, and not-to-long-ago updated their Terms and Agreements to stipulate that this business practice was unacceptable - hence why Valve pulled EA's games from Steam.
Considering Valve shackle the Steam Store to retail releases, such as Call of Duty, The Elder Scrolls, Civilisation and anything else that uses their Steamworks DRM solution - instead of simply having the requirement that Steamworks games must be available on Steam - I think it's hypocritical of them.