Dumbass Sayings: "Don't Let The Door Hit Your Ass on the Way Out"

Sometimes when you get fired from a job the boss says "You're relieved of your duties here effective immediately. Oh and don't let the door hit your ass on the way out." This is a puzzling thing to say to someone. Presumably this lady doesn't like me so why would she be giving me this helpful warning about the dangerous front door? Shouldn't she want the door to hit me on the ass and propel me onto the sidewalk in shame? They really shouldn't give this sort of warning. Of course it's not a real warning because no one can get hit in the ass by a door unless it's a revolving door and they just stand there. Obviously I'm not.going to do that. So this is more of a sarcastic warning. But generally sarcasm is to intend a joke. We certainly aren't on joking terms here so this is completely out of place and inappropriate. If you really hated the person you were firing you would say "Let the door hit you on the ass on the way out!" Or "When you leave swing the door open really hard then walk through half way and stop" or even a simple "Go fuck yourself." Now if someone said that to me I might understand where they were coming from, but telling someone "Don't let the door hit your ass on the way out" is like saying "Don't go fuck yourself." Do you see how confusing that is? This is why I don't mind being fired. The bureaucratic, excessive, overly confusing behaviors even extend all the way down to casual speaking!

Unless you work a job riding on a donkey I don't see how this saying applies.