"You can" is kind of misleading; more like "you have to". Developers/publishers use Steamworks (and other such DRM) with retail copies in order to prevent you from selling your copy to someone else. After all, if the consumer were able to exercise his/her right of resale, a black hole would appear and suck everything in, destroying reality as we know it. They're doing it for the sake of humanity!

...On a less silly note, like someone already mentioned, if a game is already available DRM-free on disc, there's really not much point in handing out a GOG key as well. The keys that came with physical copies of The Witcher 2 were redeemable on GOG, but that was to encourage people to make use of the service.

Why?
Steam killed retail in the first place.

Eight characters, expires July 2014.

That doesn't make much sense to me. The only reason games have steam keys is because they want to force you to use their drm, GOG doesn't have the drm so what's the point in giving people an extra copy?