Busybox was quickly replaced by BSD-licensed Toybox everywhere for that exact reason.
Copyleft licenses (like the Gnu General Public License) mandate that all derivative works remain free.
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).