When I was let go the HR person walked with me back to my desk; watched me shut down my computer (no doubt my accesses were already gone but I never checked); watched me pack up my personal stuff; and walked me to the door.
Was it pleasant? Of course not. But she was doing her job and I wasn't going to get upset about that. Had they made me come back to get my personal possessions I would have been much less than pleased and probably sent them an invoice for my time since I was doing work required by the company when I was no longer an employee.
Employers have to err on the side of caution. They have to protect their assets be they intellectual or physical. People who have been fired/laid off/let go can be upset and act in irrational ways. Better to be safe than sorry.