Yesterday the creators of Red Orchestra and Killing Floor series (both were mods for Unreal Tournament 2004 back in the day!) updated their EULA to forbid purchasable mods for their game.
Here’s the link to their EULA and the modding part:
While we encourage folks to make Mods, Mods will not be supported by Tripwire Interactive and its licensors, licensees or suppliers, and if distributed pursuant to this license your Mods must include a statement to that effect.
Your Mods must be distributed for free, period. Neither you, nor any other person or party, may sell them to anyone, commercially exploit them in any way, or charge anyone for receiving or using them without prior written consent from Tripwire Interactive. You may exchange them at no charge among other end users and distribute them to others over the Internet, on magazine cover disks, or otherwise for free.