But shutdown/reboot/halt could well check the pre-requisite user id (or other kinds of permissions needed for you to be allowed to reboot/stop the system) before invoking all those subsystems which all run on as if they were going to succeed when they are not.
And would it not be very weird if 2/3 managed to run, sending wall messages to everyone, preventing new logins and so forth but then not rebooting? Or not preventing new logins, not sending the message but still reboot?
All of those "corner cases", including the quoted permission denied messages, could well be prevented but are not.
Meandering errors to you are important messages to those who care enough to interpret them.