Crew rest rules. Do you really want an airline to be crewed by people who only had 3 hours of sleep in the last 24 hours?
Again, the appears to be a last minute problem. If the crewmembers were regularly scheduled to do that flight then they'd have been boarded previously. The four passengers who deplaned would never have been onboard.
FWIW, when there are no volunteers the airlines usually go to the cheapest tickets and the last to check in. So if you are buying your ticket off Orbitz or Hotwire, make sure you check in an hour or two prior to avoid being deplaned off a crowded flight.
About as much as I would like my doctor to have had 3 hours of sleep in the last 24 hrs. To have the flight the following day covered United could have shuffled their employee rosters. This entire thing was because United did not want to be inconvenienced.
And if they had gone with the cheapest tickets or the last one to check in it would be one thing. For all we know the man in the video paid top price for the section and checked in first.
Nonetheless, crew rest rules are federal law. The next morning's flight would have either been canceled or delayed by several hours. Again, better to inconvenience 4 than 130.
You are free to believe they pulled off that one man because he was Vietnamese, but the reality is that it's based on lowest ticket price and check-in time. This will come out in court if the airline presses charges against the passenger
if he sues the airline. Again, the airline's PR department would prefer this to go away, but if the passenger decides to sue, the airline will literally make a federal case out of it and that doctor will be convicted of violating Federal law.