How are mods their IP? if you add an asset, or you REPLACE an asset. In most cases that's YOUR IP. When I added a M1A1 tank to Battlefield 1942 as a kid, I made the model, I made the textures, I wrote the code for it. The fact that a user places those new assets in a directory in such a way that someone else reads and uses them doesn't mean i'm using their IP.
You're still using their engine and their world. Just because I bring a dish to someone's house doesn't mean I own it. (Of course, in this case, you own the house and you're bringing the dish.. so.. doesn't really work as well)
Of course, this goes back to what was posted last week about true ownership of products (John Deere issue). I'd say you're still right because we've already paid them for their portion of it by buying the game. Not to mention, they'd likely make more money because people have to buy it to use your content.
If anything, modders create demand for the original product. You can't play the mod if you don't own the game. Something like DayZ was a major driver of ARMA2 demand.
It's the same as making a remix of a song. The base of your new version is still the original authors work, so the author deserves a cut. A mod is akin to a remix of the game, and that remix is still mostly Bethesda assets, not to mention running atop their engine.