I don't think it's true of anything but OSX and most desktop linux-en. Solaris, hp-ux, AIX use(d) ksh, QNX too I think.
Also, I didn't know that zsh was ever the default? I thought it switched from tcsh to bash around 10.3 or 10.4.
10.2 was tcsh, 10.3 was tcsh for user shells and /bin/sh for scripts.
I think 10.4 was the zsh experiment, then 10.5+ bash. We'll see how long bash lasts being a gpl shell.
Just giving historical context, not trying to say anything about markeshare or anything. Apple seems really willing to change shells is basically all my post amounts to.