Only because reciprocity is prevented by default through copyright law. In the state of nature, I could not prevent you from modifying my published software, and you couldn't stop me from taking your published modifications and using them, or distributing around. GPL is crafted to create an enclave supported by copyright law, in which things behave similarly to the desired "natural state".
As for source vs. binary thing, I feel that if there were no restrictions on binaries, there would also be no reason to not release source code in the first place. This makes the source code aspect important for real-world GPL, but not necessarily a core issue for a hypothetical copyright-free world.
> A program is free software if the program's users have the four essential freedoms: [1]
> The freedom to run the program as you wish, for any purpose (freedom 0). > The freedom to study how the program works, and change it so it does your computing as you wish (freedom 1). Access to the source code is a precondition for this. > The freedom to redistribute copies so you can help others (freedom 2). > The freedom to distribute copies of your modified versions to others (freedom 3). By doing this you can give the whole community a chance to benefit from your changes. Access to the source code is a precondition for this.
The FSF twice says that having "source code is a precondition" for the freedoms the GPL seeks to protect. And that makes sense. If what you care about is allowing users to "study how the program works" and "change it so it does your computing as you wish," mere freedom to reverse-engineer binaries is insufficient.
Indeed, your interpretation makes the first two freedoms meaningless. It doesn't infringe copyright to reverse-engineer programs for study, or modify them for your own use. You don't need the GPL to enable those freedoms. Those two freedoms only make sense if you read them to mean that users should be able to study and modify the program in the "preferred form" for doing so (i.e. buildable source code).
Sure there is. The GPL just used copyright as a hack to enforce on a limited basis what the creators explicitly think ought to be a universal obligation not dependent on the use of some upstream content that imposes the rule, for which the natural enforcement mechanism would be for the rules to be in law directly instead of copyright. “Copyleft” essentially implies that—its twisting what proponents see as a bad law into a tool for approximating the nearly diameteically-opposed law they wish existed in its place.
> The source code for a work means the preferred form of the work for making modifications to it. For an executable work, complete source code means all the source code for all modules it contains, plus any associated interface definition files, plus the scripts used to control compilation and installation of the executable. However, as a special exception, the source code distributed need not include anything that is normally distributed (in either source or binary form) with the major components (compiler, kernel, and so on) of the operating system on which the executable runs, unless that component itself accompanies the executable.
A world where you could freely modify binaries and distribute the results would not provide the same freedoms that are essential to the GPL. Copyright is absolutely necessary to accomplish that goal.
The GPL also embodies the fundamental moral theory of copyright: "I created this thing, so I get to decide the terms on which you use this thing." The GPL doesn't just embody the idea that users should be free to do what they could in a state of nature. It is fundamentally predicated on the idea of enforced reciprocity.