Fair use. Of course you are correct, fair use does allow for people to profit off copyrighted content in certain situations, the most prominent examples are "Let's plays" on Youtube because of the mix of copyrighted content and the transformative element of the let's players additions of voice content and editing etc.
But in what situations do you see "fair use" being successfully employed in the context of modding?
As we have seen from the scene prior to the curated steam workshop nobody has been able to monetise a mod via "fair use" of Bethesda's copyrighted content. It hasn't been successfully argued. If they had then you would have seen paid mods appear far before any of this happened. I think it's fair to say that the arrival of paid mods has only come about because of Bethesda's approval. If Bethesda could shut down all mods that profited without their permission then I am sure that any modder who wished to prevent their content being used for-profit could do so as well.
Of course feel free to outline a situation where fair use can apply here. Even if you can find a way to prove "Fair use" of another's content it doesn't grant you immunity from having to deal with attempted DMCA takedowns. There have been numerous instances on YouTube where videos that are protected under fair use are removed for weeks on end due to ludicrous claims by copyright owners, the most prominent people being the victims of such abuse are Jim Sterling and TotalBiscuit. Of course the claims ultimately fail but not after time, money and mental sanity are wasted.
I'm not quite sure what you are trying to get at with your second point, I think that I need a little more clarification. What sort of abuse does open-source programming face?
I can imagine that you mean that there is generally few instances of entities ignoring the terms of licenses, but again clarification please.
Not sure if this helps but there are examples of commercial companies failing to abide by the license terms of open-source code.
I mean you are right in saying that Youtube and open-source programming aren't being abused but there are reasons for this. For commercial companies it doesn't make sense to violate GPL licenses left and right because doing so invites legal battles and the potential to be sued for millions. For Youtube we don't see much abuse because in cases where there is genuine copyright theft because the consequences are severe. In order to make real money from Youtube you need to be able to sustain a significant audience over multiple videos. I'd like to see someone try doing this with stolen content since 3 successful DMCA claims result in account termination. I have absolutely no idea about "stock resource sites".
Whereas with Steam Workshop there is a significant incentive to steal content. Unlike Youtube you can make a decent amount of money very quickly. A low barrier to entry and the ability to set your own price means that it is viable to steal content. Simply steal a few skins off nexusmods and sell for $1. All you need is for 400 people to buy and hey you have $100. Content gets claimed? Well all you need is another account and you can repeat.
I will say that we we won't see much of the second charlatan, the type of person who sends patently false DMCA claims on content they have no claim to, though I think that we will see plenty of people stealing other peoples work. If not outright stealing then people making items extremely similar to popular works which are clearly ripoffs.
It has happened a few times in CS:GO, in a marketplace where content that is to be monetised is very tightly controlled by Valve who presumably vet it themselves.
For examples of people stealing the ideas of popular products by releasing very similar products then all you need to do is go to the play store and look at the top lists of paid games. Then search that game and see how many different apps there are trying to sell a knockoff product with a similar name and price.