I don't follow. You're saying that if people who have made mods for Dwarf Fortress get together and make something cool on their own, they have to pay Toady?
There's a fine grey line. Personally, I have no problem paying someone to make Stonesense, et al. but there are some that would say even derivative works that rely on Dwarf Fortress should be approved by Toady or deemed "illegal" or immoral. It's kind of like how Blizzard claims rights to game data in the memory of your computer and any application that alters that data is infringing on their property.
My personal stance on the issue is that it's perfectly fine to pay and play someone's mod as long as they do not ship with Dwarf Fortress files. If you provide a method of downloading DF from the original source and instructions on how to set it up, I see no problem there either.
I fall in the line of people that says any data created by any application I launch that is saved to my RAM/Disk is mine to read or modify in any way I see fit. (I had to word that proper... I do not find it cool to share compiled code unless approved by the original author, but any "compiled" data that uses my CPU to compile it is free game.)
If Toady wanted to stop modding (and that's his choice) he could easily implement one of the many memory obfuscation algorithms. Currently, IMHO, he approves all "DFHack" style modding by not making an effort to stop it or providing an alternative interface to the data. That's a good thing in my opinion. It shows that he knows people want to play his game, but they need X tool to do it. It should inspire him to add similar features. He has the advantage of being able to compete with the mods. He has a better interface and knowledge of that data... so it should be marginal for him to compete.