SSHd performs actions based on username
Short answer: Thousands of perl scripts, son!
Long answer: Once you get into the large hosting game, you want everything to be done via scripting, and all your backend processes to talk to one another. Manual intervention means you lose. You want one click access to suspend, create, delete, etc. So, it's most likely not "manually" created, but someone on their side got an idea of how they want to implement things, and then wrote the necessary code to add these names to all the appropriate places, set up the permissions, etc. As most things with hosting like this, it can be done, you just need to implement it according to your specific desires.