100% it's this. Their dream is to convince the wider playerbase that mods are something that should be purchased, it's creators making money from it, Bethesda retaining all ownership and rights in perpetuity of any/all mods uploaded.

The way everything has to be monetized these days is to me pretty disheartening. This idea that a thing isn't worth doing if you're not making money from it. Fortunately right now mod communities are alive and well and free to all.