Okay, I can understand that. Unfortunately, any modding done with the construction kit is legally owned by Bethesda, so if they wanted to turn someone's mod into DLC they wouldn't have to pay them.
Hehe, don't be so sure about that.
I'm studying law in my country, so I'm at least sure about my country's law.
Copyright is an automatic right which is granted immediately as you create something. Not everything gets copyright though. It needs to raise a certain "value" worth protecting, which can only be confirmed in a court. If you alter a work of someone else's copyrighted work, your work can be protected by copyright if what you've altered has turned into a work that is "new and independent". However, if Bethesda has explicitly stated in their contracts that someone other than themselves and distributors can't make money out of the game in any way, then that applies instead of course. This would go for mods and everything.
Legally, it would be interesting to see if Bethesda's contract contains this.
In this case it's quite irrelevant though. If I understand this thread correctly, the question is more about whether Bethesda should have better DLCs from the start that modders otherwise would do later for the PC. Like Kvatch Rebuilt for instance, that should have been in a DLC from the start... that Bethesda should have thought of.