Valve made a whole bunch of announcements at GDC in the last few days, showing off the Steam Link, finally announcing when they aim to get the long-awaited Steam Machines out on sale and revealing that a new version of their Source game engine, Source 2, was upcoming.
Now more details on Source 2 have been released into the public, and Valve say that while Source 2 will be free and no cut of the profits will be asked for there is one stipulation for its use - you must sell any game made using it on the Steam Store.
Valve's Erik Johnson explained that they are making it a mandatory thing for Source 2-made games to appear on Steam, probably so that Valve will be able to see some revenue from their new game engine (anything sold on Steam has 30% of the revenue going to Valve).
Of course, this doesn't mean that a game has to be exclusive to the Steam store. You're allowed to sell it anywhere else or any way else; be it through other digital stores, platforms or in physical copy shape. It just has to be on Steam as well as those other places.
And given that Source is free, doesn't take a profit cut and gives access to a lot of Steam's in-built system such as the Steam Workshop; the making it necessary for a game to be released on their digital store really isn't going to drive many game developers away from Valve's new engine.