I am not sure I understand how it works, but if I had to take a guess it is because normally your thoughts and attention are invisible to others, but if policy

I am not sure I understand how it works, but if I had to take a guess it is because normally your thoughts and attention are invisible to others, but if policy mandates you to use body language to indicate your attention, then the employee realizes any lapse in attention will be visible from afar, so the employee who doesn't want to be seen losing attention it becomes mandatory to constantly show that yes you are in fact still paying attention...