Context:
Permissive licenses (commonly referred to as "cuck licenses") like the MIT license allow others to modify your software and release it under an unfree license. Copyleft licenses (like the Gnu General Public License) mandate that all derivative works remain free.
Andrew Tanenbaum developed MINIX, a modular operating system kernel. Intel went ahead and used it to build Management Engine, arguably one of the most widespread and invasive pieces of malware in the world, without even as much as telling him. There's nothing Tanenbaum could do, since the MIT license allows this.
Erik Andersen is one of the developers of Busybox, a minimal implementation of that's suited for embedded systems. Many companies tried to steal his code and distribute it with their unfree products, but since it's protected under the GPL, Busybox developers were able to sue them and gain some money in the process.
Interestingly enough, Tanenbaum doesn't seem to mind what intel did. But there are some examples out there of people regretting releasing their work under a permissive license.
Controversial opinion: Copyleft is actually more free than permissive licenses.
Because the way the GPL works is how the world would be if there were no licenses and no copyright at all. Because then anything made public is free to use. And if I were to reverse-engineer a binary then I could still add that code to my software.
But since we live in a world where we play make-believe that you can make something public and still "own it" at the same time (e.g. copyright) and where using reverse-engineered code can still get you into legal trouble, the GPL is using their own silly logic against them (like fighting fire with fire) to create a bubble of software that acts like a world without any licenses.
Permissive licenses don't do that, they allow your open software to just get repurposed under a non-free paradigm which could never occur in a world with no licenses. And so ironically permissive licensing in a world that is (artifically) non-permissive by default does not reflect a world with no licenses, and is thus less free than Copyleft.
That's actually an important factor for ancient software whose source code was lost. A developer could, for example, declare all their old Atari 2600 games to be under GPL by just announcing it in their news blog. Collectors could then hunt for the binary files and decompile them. Decompiled software is still a derivative work, so that source code would still be under GPL. Sadly I'm just aware of one case from years ago where I can't even remember the specifics who and which software it was but he was like "I found some floppy disks from the 1980s, I lost the source code but binaries under GPL, so have fun".
Well you've convinced me. I always thought copyleft sounded cool, but never thought of it in this way of more/less free