This was already the case for Skyrim and presumably Fallout 3 and New Vegas. It's in the EULA:
https://www.reddit.com/r/truegaming/comments/3fis0s/why_specifically_is_it_not_legal_to_sell_mods_for/Specific section:
http://pastebin.com/q4LVsAnrEntire EULA:
http://store.steampowered.com/eula/eula_202480Essentially (assuming the EULA would hold up in your local court, not necessarily true especially in Europe), the modder cannot sell mods themselves. And Bethesda has the right to distribute them as they see fit.
Technically this only applies if the modder used the official tools... But it's probably against the license to backwards-engineer the ESM files. So that's not an out.
Asking for donations is fine, and participating in Bethesda/Steam's marketplace was fine. But yeah... Bethesda does essentially own the mods, or specifically, has the rights to do whatever they want with them.
This isn't new though...
It's also not *bad*, it's not like Bethesda is going to steal mods and sell them themselves. As we saw, they offered a system for people to sell mods... or keep offering them for free, whatever the mod author preferred. They only ever offered mod authors an optional system.
Technically Bethesda could have just put all the mods up for sale and taken 100% of the profits (split with Steam I guess). But to be fair to their detractors, that would have been a terribly short-sighted decision.