Can't share the boot drive root dir with authorization for "everyone" set to "no access"?
It's not innocuous. The permissions you're seeing (and controlling) in the Sharing preference pane are the regular filesystem permissions, not some separate sharing-only permissions. Setting the permissions here to Everyone: No Access would deny all users (other than root) access to the entire volume, thus rendering your Mac unbootable. This seems like a good enough reason to prevent you from doing it.

You could ask why the Finder does allow you to do this, but I don't think there's any particularly deep answer. It's impossible to block all possible destructive actions, so inevitably Apple will have chosen some to block and not bothered with others.