The GPL does state that the code must be freely available whenever it is made available to the public, and people who make addons and scripts for use with Blender don’t really have a choice. The BF was actually wrong about the idea that you get licensing control over your scripts because it’s not officially considered output or ‘data’.
In a sense, this is one reason why the only real direction for Blender as a viable application is to provide the entire 3D pipeline in one spot (because the only time you can have a truly robust I/O system is when it connects to another free application). I know that there are some alleged workarounds but such a thing either needs to be a system that has to be developed first or is something that has a bit of legal confusion with it.
The only way you can have a truly secure system where people have to buy your plugin is to either abandon Blender and start making plugins for Modo or Maya or make your own standalone application that uses a GPL-compatible format for import and export. This is something that even a fork of Blender can’t fix due to how difficult it would be to re-license and the only other way is to make a new open source 3D application from scratch.
All in all though, the fact is that a lot of people in the FOSS community are notoriously cheap and will get something for free when possible, if you want big money in plugin development then you need to just forget about FOSS.