GPL is about ensuring that all users of the code, both developers and end users, receive the same freedoms that the authors of any derivative work had.
But just as the GPL author has the freedom to make their code available, another author should be able to choose not to make their own code available. Again, this has nothing to do with freedom, this has to do with imposing your personal beliefs on others.
Yes, the other author has the ability to choose to not open source their code. But they absolutely do not have the choice to not abide by whatever license was applied to the libraries and original works they are building from.
The freedoms awarded by the GPL are not intended for the authors of the derivative works but for everyone else. The restrictions in place only prevent the author of the derived work from restricting everyone else.
