Earlier quoted context omitted.
I don't understand why GPLv3 would be a problem on macOS. On macOS, you can run anything out of the box (depending on security/gatekeeper settings) and you can replace or modify system executables (such as /bin/bash) if you turn off SIP (which is slightly tricky to do but can be done from recovery mode.) Running your own customized /usr/local/bin/bash is easy of course, and package managers like MacPorts or Brew make…
Did you notice how many qualifiers you had there? The trend on macOS is clear: harder and harder to run/do anything that isn't blessed by Apple.
I wouldn't even classify turning off SIP as "slightly tricky". You boot into recovery mode, open the terminal, type in two words, and press enter.
Besides, this is separate from the GPLv3 question. You can absolutely recompile bash and replace macOS's version with your own, so I don't understand why this is a problem for Apple.