Seems fine to me.
Some people will still want to mod for free, others may want to sell what they make.
I'd never pay for a mod or extra unofficial stuff personally and think donations are a good way to go, but if someone else wants to sell what they made, then it's up to them as long as they comply with the rules or whatever.
But do you not think that the guys doing it for free are going to become scarce on the ground now? They are going to have to start copyright their work for a start in some way as you can be guaranteed the bottom feeders will pick off the good bits to incorporate into their paid for mods. It's all going to become more hassle than it's worth kind of thing for the guys doing it for free AND if this proves even slightly successful for Steam or worse a disaster and they ain't making the revenues they thought they would you can bet your life they will try and squeeze the free modders out.