It's a little tricky. Users can't sell Cabbage but they can freely distribute it. They can sell their instruments, but the code is not currently bundled into the binaries. Therefore users must also be willing to distribute their Csound code. If enough people were interested I could look into a way of bundling the Csound code, but it would need to be licensed as open source unless I purchase a commercial license from JUCE. Therefore, people can see how Cabbage does it in the source code. On the other hand, only developers themselves would be able to follow the C++ code and figure out how it all works, and if they go to that much trouble, maybe they'll just start contributing to Cabbage.
As it stands, if we want to sell Cabbage plugins without violating the terms of the GPL. they need to be open source. If enough people were still interested we could look at getting a commercial license for JUCE. But it just got costly. I have to say, this is merely my understanding of things. I could well be wrong. Either way I'm happy to discuss whatever options you may think of.