Personally, I don't see a problem here. Bananian was my first contact with zsh. I only knew bash before, but I didn't encounter any difficulties with zsh.|
You're talking about beginners. While I don't think a beginner should or would buy a Banana Pi and install Bananian on it, what is it that a beginner does on a shell? I guess, entering a series of commands. And that would be the same either on zsh or bash. I'd expect more differences when you do more advanced stuff like scripting. But with every script, you can just add the shebang line and make sure you're script runs in bash and work as expected.
Plus, as you mentioned, anyone can easily change the default shell and it's documented on the Bananian website. Sure, if it's integrated into bananian-config it's more convenient and I wouldn't mind it. But I don't really see it as a necessity, because I'd expect anyone who tries Bananian to have some kind of basic linux knowledge.