There are other reasons for walking an employee out the door, for example someone like me with complete and total access to all of multiple companies' data. If I was terminated and got mad about it, I could dump a bunch of PID and sell it, publish the companies' trade secrets or start mangle data, rendering it useless to the company. I could destroy backup files and drop databases, putting them potentially weeks behind as they try to recover, rendering POS systems useless, etc. Someone like that you escort out of the building and give them a lavish severance package if you lay them off. Even if I was to serve notice that I was quitting, I'd immediately have my access revoked, for the same reasons. They don't want me angry with kind of access.