So they could even post it on a public site for everyone?
If they are just paying you to make the addon, instead of paying for a license to use it like when you buy a commercial game, that isn't so bad.
The thing that I am against is selling them as if they are an expansion pack/full game and having people buy the rights to play with them. Only the makers of the game really have a right to do that as far as I know.
I guess if someone just paid me to work on something and then I/they could just release it for free it would be fine
Still, I wouldn't pay someone for that. But if someone else wants to, so be it.