Valve's corporate structure is quite... different than most companies. Their company is also pretty unique, as is their money-making engine. They have a tiny number of employees for how huge their platform is (and the growth they want to see), those employees get paid quite a bit, those employees also aren't structured in the usual -> goon -> management -> useless management -> more useless management -> GabeN like other IT companies. Steam gets away with an outdated platform because they can. Origin and Uplay are both almost better than Steam from a functionality and smoothness standpoint, they just lose on their limited market and for trying to shove themselves down gamers' throats through exclusivity.
This is an idea that, implemented in a different fashion by perhaps a different company (or an alternate-universe Valve) could be a great system. It is NOT a system that has been well implemented and the knowledge people have of Valve makes it seem like they won't be able to manage it properly, if at all. The top 5 paid mods are all parody / protest ones so at least they're not censoring the complaining too much yet.
It's easy to see where this idea could go in a negative spin or a positive spin. The problem is it doesn't look like Valve took either scenario into consideration. It was a project someone came up with, a team worked on, and is now live. It went through the ropes of all major Steam changes but those have ended up being poor designs before (Greenlight).
I'm not against having the option to donate to modders for their work, if they so chose. But the implementation is so, so wrong.