German case only stated that a customer may sell a software license on. They said nothing about publishers having to facilitate that sale. So yes, you are allowed to sell your steam games on. But the practicalities mean that you are unable to.
Sort of like how you are allowed to sell your tattoos to anyone else, there's just no way to actually do it and the tattoo artist is under no obligation to remove the tattoo from your arm and put it on your friend's.
EDIT: I don't think the courts will force publishers to make digital resellable. If they do, publishers will jump to an MTX Free to Play model all the quicker. I could be wrong though and Valve may just be getting ahead of the curve, they might think this is a good idea. If you can share you game with others one at a time then as Dean says it could encourage further sales. I do honestly believe that people will pay for games if you give them the option, this is just a way to introduce them to a game and then let them show their appreciation by paying for it afterwards.