Lunch breaks are required by law, but they are not required to pay you when you take them. So when you work an 8 hour day, you are actually working an 8.5 hour day (8:30 - 17:00) with your .5 hour break at some point in the middle. The joke is basically the guy asking to work 8 hours straight and leave at 16:30 instead of 17:00 and management tossing him out a window.