Because modders are using Bethesda's copyrighted intellectual property as the base for their mods. Bethesda deserves a cut because it's their IP.
That seems to leave a pretty huge area for a license that requires payment.
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.