jiangshanruhua: I chose support GOG because I love DRM-Free's world.
But yesterday I change my mind.
Because I support DRM-Free so I haven't purchase any Steam's game.
Why can't GOG user get a Steam copy?
it's not fair.
using an old Chinese saying to express my feeling
人不患寡而患不均
GOG and Steam are competitors in the digital games distribution industry. What you're asking for is basically buying a product from Store A, then asking to get that product from its competitor Store B for free as well. That's not going to happen - Store B doesn't even have the right to make that decision! You would need to negotiate a deal with the company that created the product you're buying: in this case, the game publishers.
GOG has basically done what you're asking, but in reverse. They've negotiated deals with some of the game publishers so that if you buy the game on Steam, you can get it for free on GOG as well. (Yes, it's not what you wanted, but the idea is the same.)
You can buy games on both GOG and Steam too, you know? You don't have to restrict yourself to a single store. (Though some will disagree, I know it!) There's no rigid doctrine to supporting DRM-free gaming. Just do what you think is best for you as a consumer. =P