> Now if someone takes your permissively licensed code and makes it proprietary, THAT is a real hindrance to users of that software

I look at it as decent screening process. If the only reason the person / company is returning the changes is the license, then it is very probable that I don't really want those changes or to work with them.

Its also a really poor business decision on their part (yet another good reason to not deal with them). They will spend time on their fork and have no influence over the direction of the code.