That's not going to stop people clogging up their CS system as they angrily fight over it.
"My great-grandfather did not travel across four thousand miles of the Atlantic Ocean to see this nation overrun by immigrants. He did it because he killed a man back in Ireland. That's the rumor." -Stephen Colbert
Yeah, we've seen that happen in other games & industry but I forget the specific examples at the moment. WoW had something the CS reps always had to deal with even though it was 3rd party. (e-bay transactions and something else...) I remember seeing somewhere Apple CS gets questions/ complaints about apps all the time, too.
People pay for it, they expect it to work 100% (even if it's their fault it's not) and will bitch to someone at your company when you're the face of the product.
The past cannot be changed. The future is yet within your power.
This is a good idea in theory and will likely work as intended when introduced with a new game. People have an irrational hatred against paying for things they got legally free in the past, I guess Valve underestimated that factor and are now in shock about the reaction.
The support nightmare they will have once the "bouncing elf titties" mod for Elder Scrolls 6 doesn't work anymore because Bethesda patched it and the modder moved on to different things will likely be their next shock.
Hope it works out, though. Getting good Mods by paying the modders their fair share is in every gamers best interest. If that fair share is 25% like Betheda thinks or more because they already sold more copies than they would have otherwise due to their game being modable is an entirely different discussion.
Here's a very good piece about how this should have been done.
TL:DR - Asking users to pay for mods with no quality control of those mods, no assurance that they wouldn't break anything else or become unusable after future patches or were actually any good was Valve & Bethesda asking for money for nothing. If Valve/Bethesda had devoted resources to it and ensured that the mods offered for sale were of decent quality and would be fixed if future patches rendered them unusable, it would have gone a long way to smoothing this idea over. I.e. mod developers deserve to be paid for their efforts but Valve & Bethesda etc. have to do some actual work in order to get paid. Also asking users to pay for fixes to things broken in the game by default (UI) is stupid.
"Bourgeois society stands at the crossroads, either transition to socialism or regression into barbarism" - Rosa Luxemburg, 1915.