If you leave the decision in their hand they won't make the correct business decision, but one that gets them out the door quicker.
I would argue two things on that:
1) Having six employees work an additional ten minutes, thus be paid for ten more minutes each, will cost more than the profits made from serving two extra cars during that time.
2) If the correct business decision is to stay open in order to capture the money trying to be given to the business, then it is actually the owner who is making the wrong decision by closing at that time and not the employees.