Wow, Bethesda is sure greedy in a self-punishing way. I mean you have to buy the game before buying mods. Surely 5-10% would be enough.
They said something along the lines if "25% is an 'industry standard". I think they referenced to the hats, knives and whatnot being sold on TF2/CS:GO marketplaces.
I'm sure from Bethesda's perspective, they own the IP and built all of the tools for the modders. It made sense in their meetings, but failed in the community.