Are parks “proprietary”? I can’t run my car dealership from one, so it’s …proprietary? No. So using the terminology of “proprietary” doesn’t do justice to what it actually is.
If I use GPL'd code, I have to keep releasing my modifications for free because it's mandated. I have to do that even if I do 1000 hours of labor but they gave me 30 min of it. So, it's also near-infinite work required in return for finite work they did. And I have to bind others to this with my own work.
That's not someone giving me a gift. I'm not sure what to call that except a collective work with permanent obligations for all parties. It's more like a job or corporate charter or something. I like another person's claim that it's creating property with requirements for those using it (which copyright certainly does).
Pedantically, only recipients of your updated binary are owed updated source, so if you're not distributing that binary to the whole world, you're not required to release updated source to the whole world. Kind of a nitpick, but maybe not.
The point of copyleft is that the author wants the code to remain available to the public, not to give it as a gift to whoever wants to build their own closed source system
The obligation is not to the author of the code, it is to the public. MIT-style licenses are gifts to people and companies who produce code and software, copyleft licenses are gifts to the public.
I don't give a shit about the happiness of programmers any more than the happiness of garbage collectors, sorry. I don't care more that you have access to the library you want to use at your job writing software for phones than I care that somebody has access to the code on their own phone. You're free to care about what you want, but the pretense at moral superiority is incoherent.
It is non-negotiable. GPL is basically proprietary software. It's owned by the public, and all of the work that you do using it belongs to the public. If you steal it, you should be sued into the ground.