It is pretty shrewd, valve offers something useful for free to devs. But then it makes using their online store application mandatory to play the games. Then all those players see those juicy specials on games and eventually start buying. Once they have several games on Steam, they want to buy their new games on steam as they then all have it on the same place and it feels much more convenient. Not to mention that most PC gamers like steam.
As a result, more and more gamers insist on steam, so they will only buy outside of steam if they still have a Steam Key in return. For this the game needs to be SteamWorks, so that is yet another layer of incentive for Devs to use steamworks as it allows them to sell the game themselves, skipping all the third party fees while still providing that Steam Key that the end users want. Resulting in even more devs using SteamWorks and then even more traffic on Steam.
Really a fine example of applied psychology.