There have been a few other games like that, that only the DRM-free version asks for an authentication key (Mobiloid, Gimbal and Spacechem come to mind) - regardless, good question...
Good point, I remember noticing it that with Mobiloid. I haven't tried installing it yet (through Desura or DRM-free) so it might be that.
Has anyone else tried downloading the soundtrack for McDROID? I have done several times now and WinRAR has consistently said it was corrupted.