This is what King Kong does with Cease and Desist letters:
In all reality they have no claim to this. If the mod team could get a good lawyer they would win easily, and that's only if WB was stupid enough to take them to court. It's a shame they're just going to roll over and let WB walk all over them. In both cases (Skyrim and TATW) the mods are free to use, so the modders are not earning money off of copyrighted material. There may be some small claim that the Skyrim mod is taking money away from other LotR related sales, as some games already exist that are similar to Skyrim (The War in the North perhaps) but I think this would be hard to prove in court and a judge most likely wouldn't go for it. In the case of TATW, there is no turn based LotR strategy game that WB is marketing that TATW would be in direct competition with.