Honest question- why should Bethesda get any money from mod sales? You need to have purchased the game to use the mod, and mods add something to the game experience that Bethesda didn't have to pay for themselves.
A lot of people in this discussion are asking this question, but I don't think anyone's hit on the real, fundamental answer.
The real, fundamental answer here is that Bethesda can just shut off modding entirely, or at least, they can going forward. Legally, this is their playground up to and including the ability to kick everybody off unilaterally and suing anyone that doesn't comply. Therefore, they can basically charge whatever they like, and give you the choices of taking the deal, or getting nothing.
Now, there are many ways of looking at this situation and you are free to use other value systems for your own opinions. I'm not saying this is the only way of looking at the situation. I'm just saying this is in some sense the most fundamental way to look at the situation... we wouldn't even be having this discussion if it weren't for this base truth of copyright law, EULAs, and the DMCA. Also, I'm not advocating for this position, just explaining it. I have neither created nor necessarily endorse this situation.
("The power to tax is the power to destroy"... well, it runs the other way too.)
"If You distribute or otherwise make available New Materials, You automatically grant to Bethesda Softworks the irrevocable, perpetual, royalty free, sublicensable right and license under all applicable copyrights and intellectual property rights laws to use, reproduce, modify, adapt, perform, display, distribute and otherwise exploit and/or dispose of the New Materials (or any part of the New Materials) in any way Bethesda Softworks, or its respective designee(s), sees fit. You also waive and agree never to assert against Bethesda Softworks or its affiliates, distributors or licensors any moral rights or similar rights, however designated, that You may have in or to any of the New Materials."
And the real, fundamental consequence to disabling mods is that two things happen. First, brighter-hat modders abandon the franchise to make mods to some other game that allows mods. Second, darker-hat modders crack the game, restore the modding capability, and distribute mod-enabling patches or pre-loaders for every official release. The black-hats will simply distribute pirated pre-patched copies, with pre-loaded mods.
Thanks to NexusMods, much of what I actually see when I play Skyrim has actually been created by other end users. This includes the "killer app" of mods, which involves the physics engine and more detailed model skeletons. You know which one I'm talking about. Jiggle, jiggle. The studio probably couldn't create half of those mods and still sell the game on console platforms. They would either bump up against resource constraints on the device or licensing restrictions on content (lookin' at you for this one, Nintendo).
The ability to mod is almost intrinsic to any game where the physics and rendering engine is separated from the content. The only question is whether that content-loader is exposed in any meaningful way to the end user. Even Doom had its WADs. Unless you're willing to completely break your world-design paradigm, motivated players can find a way to alter the content.
So Bethesda does not actually have that power to shut off all mods. They only have the power to shrink the brighter-hatted mod community. And they shoot themselves in the foot by doing so. They couldn't sell their own downloadable content expansions as easily!
Never mind the "bug fix" and "cutting room floor" mods. Arguably, these are things that Bethesda should have done themselves, prior to release.
I can think of no reasonable argument why Bethesda should get a cut of any mod. It would be like the construction contractor that built the wall getting a cut if Banksy painted something on it. Or maybe it's more like the original artist of a thrift-shop painting getting a cut when someone paints dinosaurs into it and resells it for more money. It makes no sense.
Copyright would only apply in that instance if someone made prints from the altered painting, but if you have to buy that existing copy of the original to get the additional dinosaur, there is no copyright violation, because there was no copy made. The dinosaur may now be inseparably attached to the original, but the original was already paid for when it was altered.
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.
I think there might be a positive market dynamic overall for the original game makers to keep a cut: it would encourage more games to be made moddable, especially indies, who would look to an actual extra revenue instead of the unverifiable "mods sell more of the original game", which is definitely true for big names like Bethesda (whom I think could do well with a 0% cut to encourage even more professional mods), but might not attract so many new people into buying an indie game.
The real, fundamental answer here is that Bethesda can just shut off modding entirely, or at least, they can going forward. Legally, this is their playground up to and including the ability to kick everybody off unilaterally and suing anyone that doesn't comply. Therefore, they can basically charge whatever they like, and give you the choices of taking the deal, or getting nothing.
Now, there are many ways of looking at this situation and you are free to use other value systems for your own opinions. I'm not saying this is the only way of looking at the situation. I'm just saying this is in some sense the most fundamental way to look at the situation... we wouldn't even be having this discussion if it weren't for this base truth of copyright law, EULAs, and the DMCA. Also, I'm not advocating for this position, just explaining it. I have neither created nor necessarily endorse this situation.
("The power to tax is the power to destroy"... well, it runs the other way too.)