If they had recognized that they needed 4 seats for crew before the passengers scanned their tickets and walked down the jetway as is usually done they wouldn't have had a problem.

They could have started the process before the flight booked, asking for volunteers before they got seated and attached to the idea of being on that flight. If they had no volunteers, they could probably have randomly selected passengers to deny boarding, and rejected them at the gate. While I am offended by them violently dragging the man out of his seat and off the plane, if the ticket scanner doesn't blink green the passenger likely won't force his way onto the plane, and if he was injured by security while doing so, public opinion would not be so united against the airline.