and most importantly editors/distributors don't have a leg to stand on when requesting 75% of the money on the premise that they sold the engine
The claim that without the game itself there would be nothing to mod seems like a rather large leg. Never mind that Valve is allowing the use of their store and payment system which makes it a lot easier to collect money.
That said, there are far too many problems with the current implementation for Valve to allow for paid mods. They already have enough issues with quality control on Greenlight. Mods are an even bigger problem as there's no guarantee of quality, indication that there won't be conflicts with other paid mods, and the invariable jerks that submit existing free mods that aren't theirs in order to make a quick buck.
If Valve wants to do this they should develop a system that makes it easier to address some of those issues before they try to offer paid mods.