The article mentions it is because Bash went to GPLv3 and Zsh is MIT, and Apple is (my words) allergic to GPLv3. They avoid it everywhere. (Which tells me its a fantastic license.)
> Why is zsh forced on users anyway? Are there known issues with bash?
ZSH isn't GPL which may be important in the smaller non-Linux part of UNIX-like world. I would very much like to see ZSH in FreeBSD's base system already.