Actually, I would say that the "difference" is that United boarded the entire plane, the tried to get four people to give up their seats. Overbooking happens; b

Actually, I would say that the "difference" is that United boarded the entire plane, the tried to get four people to give up their seats. Overbooking happens; but, in my experience, it's dealt with after checking in and before boarding. They make an announcement and ask for volunteers before anyone has boarded the flight. In this case, United boarded the flight without considering the flight crew they needed to put on the flight -- putting them in the situation of removing four passengers.