Try this spin: if you release a program as FOSS with the GNU license, why should anyone respect the licence?
Same with movies/music: before release, nobody has a copy. It is then released subject to terms you may or may not like; nonetheless, those are the terms and if you don't like them you have no right to disregard them & make a copy anyway.
It's not like smells. Software, be it programs or movies, does not diffuse by itself to random passers by. Copies are deliberate, and each is subject to contract law.
The GPL is a hack. If copyright goes away, you could just copy, disassemble, or duplicate the funcionality of a piece of sotware in order to learn how to tell your computer to do exactly what you wanted. DRM would go wild and hardware oriented, and a constant arms race would follow, but DRM may be an impossible problem.
For example, I'm producing a closed-source, proprietary piece of software to make a living. I will aggressively defend my copyright. Do I believe in copyright? No. But I'm operating in a bad system and I need to eat. In a better world, I'd be doing the same thing as work for hire for a group of actors within the industry who would collectively benefit from it, and customizing for any who had particularly unique use cases. I'd be working to reduce friction for the entire system and to make the tools that computers are generally more useful, rather than reenforcing the oligarchies of the market leaders who can pay my price (which is almost completely unrelated to my effort, but rather to the value of advantage over competitors in an underserved market.) My software being proprietary is just limiting general productivity gains and distorting the market (from whence comes it's value), and my enforcement is just limiting everyone's freedom to use their own machines.
another tl;dr, the GPL hack creates an island of freedom within a restrictive framework, and if the restrictive framework is gone there's no need to respect the GPL, a child of that framework.
I think many creators of GPL software see it differently, and wouldn't be ok with people making proprietary modifications to their code, even if anyone was free to disassemble the modified code and try to make sense of it.
I know many creators of GPL software would absolutely see it differently, but they still wouldn't get to tell me how to use my own computer. Restrictions on your freedom to express yourself is what the GPL was designed to fix, even though it itself is a restriction on your freedoms.
Same with movies/music: before release, nobody has a copy. It is then released subject to terms you may or may not like; nonetheless, those are the terms and if you don't like them you have no right to disregard them & make a copy anyway.
It's not like smells. Software, be it programs or movies, does not diffuse by itself to random passers by. Copies are deliberate, and each is subject to contract law.