Compatibility with older devices that aren't eligible for recent OS updates, I would assume?

Or, if the person is like me, they're more comfortable in Bash after years of it being the default. Hacking around in a less-familiar shell seems like a bad idea if it's optional when deleting data, regardless of what's currently the default. (I realize that zsh can execute commands with Bash syntax, that's not my point.)

zsh has been available in every version of macOS since 10.0 - there's never been a version where bash was installed but zsh wasn't.