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.
Busybox was quickly replaced by BSD-licensed Toybox everywhere for that exact reason.
This is false. It's perfectly legal to take GPL-licensed work, modify it, and sell it. As long as the work itself does not reach the general public, you don't need to release it's source code to the public (e.g. your work for the military, you take money for your work, and provide source code to them, but not release it publicly).
Busybox is very much widely used though?
And how is it false that the GPL makes software remain free? Read the free software definition, it's about the freedom of users, not the freedom of people who aren't users (that doesn't really make sense). Free software isn't "source code available to general public even if they aren't users".
It's false that you cannot sell GPL-licensed work.
Ok but I don't see how that was ever in dispute?