End corporate personhood. IP laws have become malign corporate distortions of reasonable artist protections. No company should be allowed to 'own' anything, but certainly not IP. A company has no intellect, thus cannot have intellectual property.

As for this case in particular, selling a mod is absolutely not infringing on the IP of the game it is attached to any more than selling a clip on bookmark/magnifying glass/book light (a tool attached to a piece of media to grant the user a useful extra functionality/greater ease of use when interacting with that piece of media) infringes on the IP of the author.