In a nutshell, this is the correct answer. Going all the way back to when Apple's Java was deprecated and eventually removed from the shipping OS. In the moment, it's annoying, but I would rather not have to fight with preinstalled software when trying to get a newer version of (say) zsh installed.

Here, they not only realized it had become popular, but GNU licensing gave them an excuse to switch the default without much thrashing about.

The fact that I can `chsh -s /usr/local/bin/zsh` and macOS doesn't freak out is quite nice.