You should be, what's the difference between selling modification to other things that people do all the time. Cars, instruments, music, phones, software and 5 million other things.
They are not selling their work, they are selling their own work, they are not selling the game with the mod, their are selling a mod to the game.
It's just that gaming companies are dickheads and want to have exclusive rights to having a market around their products.
Of course you get to complain, why shouldn't you complain if you don't like something and think it's unfair. Wether it's legitimate or not you should complain, then if someone listens or not thats another question.
You don't have to buy mods, dude there are plenty of mods that are free, but if someone wants to sell it why not. The only thing that will happen is that you will have more quality mods, paid or free. Then there's the question if you have exclusive rights to a mod, I would say no, anyone should be able to copy your mod.