[int_19h]

When someone releases their software under GPL, they still retain certain rights rather than granting them to the recipient - notably, the right to sell it commercially without providing the source, and without requiring the buyers to adhere to the terms of the GPL. This is clearly a valuable right, seeing how many companies have business models that are built around dual-licensing GPL'd code for commercial proprietary use.

Strictly speaking, the only license that doesn't have the copyright holder retaining any rights is the lack of one (i.e. releasing to public domain). If you're not releasing to PD, that's necessarily because you want to retain some rights. And then it's just a question of which ones. GPL has one answer, something like MIT has another, and Commons Clause has another still. I fail to see what makes some of them proprietary, while others are not.

From my perspective, if I can get the code, hack on it, and release the changed version to others who can also do all of these things in turn, that's enough to make it non-proprietary already. Proprietary is when the software is closed source outright, or the source is provided for "educational use only" (i.e. no derived works allowed), or when derived works cannot be redistributed. Licenses that allow redistribution of patches, but not original code with patches applies, would be the grey territory.

[atq2119]

> Strictly speaking, the only license that doesn't have the copyright holder retaining any rights is the lack of one (i.e. releasing to public domain).

This is a bit nit-picking, but releasing without a license is pretty much the opposite of releasing to the public domain.

If you release without anything, the raw unmodified copyright laws apply, which are rather strict and give the recipient basically no rights - certainly no right to redistribution.

You have to make some kind of explicit statement if you want to release something into the public domain. That's why things like CC0 exist.