I doubt that this is on Valve's part entirely, surely Bethesda gets a cut of this as well, probably equal or more so than Valve. I agree though, this is a bad system; it's not truly "pay what you want", and the copyright implications of users uploading someone else's work and charging for it are going to be a massive headache.
Seeing as the Workshop is one of the easiest ways to browse, download, and install mods (not requiring any other 3rd party mod related programs, such as installers, organisers, etc.), it would have been cool to be able to "tip" mod creators with the left over money in my Steam account Wallet, given how much of a market there is on Steam now given the trading/selling of cards, games, hats, items, etc.
I would happily tip a few of my favourite mods, I dunno, a pound for example. Sure, they only get 25%/25p, but it's a nice gesture for their work, and it's not like they were making direct money off it before hand.