Makes sense, you sure wouldn't want to risk a lame duck employee doing any to the food or scaring off customers. It's the same in IT with programmers, a lame duck programmer can do a lot of damage in a small amount of time...that's why they remove all of their system access even before breaking the news to them.
Its not just IT programmers...
Whenever anyone at my company is fired, there is an appointed time to summon the employee to the boss’s office...and as he walks to the office, an IT guy remotely disables his password.
Its always the same IT guy...so you get awful nervous if you see him talking to your boss or hanging around your department too much.
In many, if not most restaurants, employees handle CASH.
Many places when you get fired without notice, security takes your employee badge and walks you immediately to the door.
They are afraid you will steal or cause damage. Not an unfounded fear.