Trilarion: Amazon sells Civilization 5 or Empire Total War both of which require Steamworks in a box, but actually the only valuable thing they sell is the Steam key, the disc is just to put in the shelf or to bypass initial downloads.
Navagon: But then Amazon never boycotted Steamworks titles, nor is in direct competition with Steam.
I think the retailers are kind of in a very close competition to Steam or D2D. They both sell the same games. For some of them the difference is, that the retailers offer disc check based DRM, while the download shops offer activation DRM. For other games, even the retailers sell the activation DRM games, so they basically just sell the serial number and the disc is purely ornamental. Then its almost exactly the same. As long as they make profit with every sale of a serial key, its okay to do it. As a side effect, customers don't want to change stores too often, so its always good to bind a customer and offer him everything he might want. Only amazon really could cut down on shipping discs and do it the D2D way.
Btw: amazon seems to hire programmers for a shop system like Steam. Seems interesting to me:
http://gamersyndrome.com/2010/video-games/amazon-hiring-for-digital-distribution-service/