If they feel like it deserves to be paid for, then they are correct. It's improbable that you can't afford to pay a small amount for a quality mod. And if you can't, then there's always the free version of the mod to play if you think that it's not worth it "just because it looks all shiny". You have no right to complain that you can't get it for free. Even less in this case, because you CAN get it for free.
The idea of commercial mods is a very good thing. They expose the mod community to much more advertisement and interest, and mod creators put more effort into their work because they know they will get some money back out of it. It increases the size and output quality of the mod community in general, and helps hobbyist game developers on their way to a proper industry job. (On a related note, if any member of this site pirates any commercial mod, I will instantly and irreversibly ban you.)
Also, Steam is a publishing service, it's only fair that they get a certain percentage of every sale. A Steam release would get far more sales, advertisement and exposure than any attempt to self-publish. There's also the fact that Steam would take care of all hosting servers, online purchases, and the like. And of course there's Steamworks and Steam Cloud and all the other fancy Steam things that go along with a Steam release. And you think that Valve don't deserve anything in return for that? They're not "making money based on other people's work", they're making money from a quality service provided in exchange for a percentage of sales.