I'm not sure I see how that's true. The GPL puts a legal onus on modifiers to publish their modifications to the source with distributions of the software. Without copyright, any other party might be legally entitled to use and redistribute said modifications, but the modifier wouldn't be compelled to publish them.

That's not really the problem the GPL is intended to fight against. The source code can usually somehow be recovered via decompiling. But that's illegal in the presence of copyright, and even reverse engineering is a legal minefield further encumbered by patents.

> source code can usually somehow be recovered via decompiling

Absent IP, ceteris paribus, you'd run as much code as possible on your servers and obfuscate what you deliver to a client. There would be an entire industry in producing technical DRM.

That's what the AGPL is designed to cover, and I think it's a major reason why a lot of software nowadays runs in the cloud. This is already a common way to circumvent the GPL.

Obfuscation only gets you so far. But faithfully recovering the original source code is not really the point. Even heavily obfuscated code is useful and can be worked with since in the end the program is still doing what it's supposed to do.

This is already possible with GPL and copyright, and companies do it. Hence the SSPL.

Ok, so why is are GPL licenses so popular compared to BSD and MIT licenses? Why did the GNU foundation even bother writing a license at all?

Those other licenses do not require republishing, so it seems that authors of OSS value the additional republication requirements provided by the GPL license. Even in under the current copyright regime, one can (perhaps other-than-legally) decompile binary to recover source-like code, and then launder and reinject the learned improvements back into the open source project.

And the GPL license does not universally grant patent rights back to the source project.

The primary difference between GPL and OSS (BSD, MIT et al) is one of philosophy.

The FSF believes that closed-source software should not exist at all. Their license is explicitly designed to make it completely incompatible with closed source software. Updates to the license (GPLv3, AGPL etc) are specifically designed to close loopholes which closed-source companies were exploiting.

OSS by contrast sees Open Source as a public good, but lives in a world that tolerates closed-source entities. It's possible to use an MIT licensed gzip library in a closed-source program, with GPL you cannot.

OSS says "Open Source us better, but closed Source is better than nothing." The FSF says "it should be Free, or you should not use it at all."

Both OSS and Free licenses mandate that users should be able to build binaries from Source. If I make a change to the gzip library then my users are entitled to those changes. (There's no "giving back", only giving forward.)

It absolutely is a problem intended to be addressed by the GPL. That's why it is specified in the GPL that the source code means the code in its preferred form for making changes.

The idea is that without copyright there is no benefit in keeping the source code secret. It justs makes it a little bit harder for the customer to recover it. The GPL is ultimately intended to be customer-friendly, therefore sharing source code together with the build system is preferred.

It's still a form of proprietary lock-in similar to not offering data export, but it's not an unsurmountable hurdle anymore.

> The idea is that without copyright there is no benefit in keeping the source code secret.

Why not? You can decompile a cookie to try to see what method was used to make it, but Nabisco or whoever still treats that method and the ingredient ratios as trade secrets. In fact, the whole idea of trade secrets are things that are kept secret because IP protections don't cover them.

That's not really comparable; by reverse-engineering a cookie one would have to fight against entropy. The issue with software is that it's magnitudes easier to figure out any trade secrets inside of it. Reverse-engineering LLMs is much more akin to the cookie example.

Exactly what I meant. GPL requires modifications to be released. That us precisely what differentiates it from public domain.