Oh, good point. I tend to forget that aspect of the GPL. My comments had in mind only the "viral copyleft" aspect. That is, a license that grants you permission

Oh, good point. I tend to forget that aspect of the GPL. My comments had in mind only the "viral copyleft" aspect. That is, a license that grants you permission to modify & distribute modified copies, and also specifies that you have to retain that same permission in all derivative works, but doesn't require anything else. You're right that the GPL has the additional affirmative requirement to actually release source code, which is using copyright law to compel a specific action. And yeah, that does seem to come from a certain collective-good concern: GNU want people to not only legally be able to modify software, but to be able to do so in practice too, so have written some things to try to make sure that's the case (the LGPL "users must be able to swap out the library" requirement is another example). I wonder if there are any licenses that have the "viral" licensing terms but impose no other requirements. CC-BY-SA comes to mind, but isn't widely applied to software.