I think there are two possible assumptions (or axioms) here.
Suppose there's a work X licensed under the GPL. You've assumed that if I create a derivative work Y, then everyone else has an automatic right to Y. If I were to restrict Y, I'd be taking away this right, and so the GPL stops me from restricting it. In fact, it severely limits what I can do with my code, but enforces everyone else's freedom to use my code.
Let us assume the opposite: that a work does not belong to the public domain until its creator chooses to share it. Then any work X that has already been shared is fully protected (by any free software license). If I create a derivative work Y and keep it secret, I in no way limit people's use of X, so I have not violated any of their rights. Similarly, if I make a restrictive license on Y, I still haven't violated their rights. They never yet had a right to Y, only to X. In this case, the GPL is violating my rights to do as I choose with my personal work, Y.
So we have two opposing assumptions and two different conclusions: If you think a work you create immediately "belongs" to the public, then the GPL protects people's rights; if you think it "belongs" to its creator until shared, then the GPL violates people's rights.
I myself am in the second camp, so I think that the GPL does not technically qualify as a fully free software license. Perhaps the distinction could be phrased this way: I believe that a potential author of a piece of code should be able to do whatever they want with it, while the GPL is predicated upon the belief that all potential -users- of a piece of code should be able to do whatever they want with it.
But I don't think you can have both. You either have to allow restriction of the users' options (if the author so chooses), or you have to restrict the author's options.
Suppose there's a work X licensed under the GPL. You've assumed that if I create a derivative work Y, then everyone else has an automatic right to Y. If I were to restrict Y, I'd be taking away this right, and so the GPL stops me from restricting it. In fact, it severely limits what I can do with my code, but enforces everyone else's freedom to use my code.
Let us assume the opposite: that a work does not belong to the public domain until its creator chooses to share it. Then any work X that has already been shared is fully protected (by any free software license). If I create a derivative work Y and keep it secret, I in no way limit people's use of X, so I have not violated any of their rights. Similarly, if I make a restrictive license on Y, I still haven't violated their rights. They never yet had a right to Y, only to X. In this case, the GPL is violating my rights to do as I choose with my personal work, Y.
So we have two opposing assumptions and two different conclusions: If you think a work you create immediately "belongs" to the public, then the GPL protects people's rights; if you think it "belongs" to its creator until shared, then the GPL violates people's rights.
I myself am in the second camp, so I think that the GPL does not technically qualify as a fully free software license. Perhaps the distinction could be phrased this way: I believe that a potential author of a piece of code should be able to do whatever they want with it, while the GPL is predicated upon the belief that all potential -users- of a piece of code should be able to do whatever they want with it.
But I don't think you can have both. You either have to allow restriction of the users' options (if the author so chooses), or you have to restrict the author's options.