I am not disagreeing with the premise that it's fair for someone to be paid for their work. However, during the Skyrim paid mod controversy (on Steam), I learned that there a lot of situations where having paid mods did hurt the modding community and created ethical concerns.

Mods were being stolen and sold by people that were not the actual mod authors.
Mods were being sold that depended on larger, more complicated mods to function, but the payment was not shared with the larger mod.
Mods that had multiple contributors were being sold by an individual who was not sharing the money with the other contributors.
Players were concerned about being asked to pay for bug fix mods when the developer should be fixing their own game. This is of course, was not the modders fault and does not mean their bug fix mod wasn't valuable or deserving of pay, but many felt the developer should pay for it, not users.

I would also point out that it wasn't just greedy players that complained about paid mods - a lot of modders thought it went against the spirit of modding because of how it harmed collaboration in the community. Suddenly, they couldn't trust that others would not steal their work or profit from it unfairly. And, that seems like a reasonable take to me, given all the abuses that modders claimed happened in the short time that paid modding was a thing for Skyrim on Steam.