There's nothing fishy about the DRM-free version having a subset of the Steam version's features. They have made it clear since the moment they announced the DRM-free version that it won't have online multiplayer support and will be missing some other features too. They clearly stated that they are using Steamworks for the online multiplayer, the friends lists and community integration and achievements and so on and so forth. As the DRM-free version is made for people who don't want to use Steam it means any and all features which the game makes use of Steam for are not going to be part of the DRM-free version.
Again, they have clearly stated this on many occasions and there is nothing fishy about it and there is no conspiracy against the DRM-free version. It's nothing to do with worries about piracy or hacking or anything like that either. It is purely because the "DRM" they use provides the features which will be missing from the DRM-free copy.
Thank you! in other words, they dont want to maintain a central gaming server forever :)
One question remains: Will the steam copy be platform independant? so I could install it on Linux OR Windows?
In that case I would take the DRM free version as Windows, since most friends and LAN parties have windows... while only I would have Linux :)