by Akien » 01 Mar 2016, 16:39
Technically if they provide the source code and respect the terms of the GPL (notably attribution, code sharing etc, and also not linking with the Steam nonfree library), they are allowed to do this. They could even sell it on Steam if they respect the terms of the GPL.
Now, morally, that sucks. It means neglecting completely the work done by contributors here by taking from them the privilege of publishing their game on Steam themselves. Free software licenses are all very permissive (even the GPL, though the copyleft clause do add some restrictions), but the hacker customs are not so friendly to such blatant rip-off of community work.
So, IANAL, but if they do provide the source code and license text they would be fine legally. Now, if we reach out to Valve, I would not be surprised to see them take down this Greenlight entry to leave this privilege to the official STK team.