To be fair, the flight was no overbooked.
The airline decided to let non-paying individuals onto the aircraft, and remove people who had not only paid, but had been boarded. If it had all been done in the departure lounge as any reasonable organisation would have done, then it would have been all over without much fuss.
But to be clear, the people who were doing the gazumping, were not paying passengers. The airline cannot really use the overbooking excuse really - just one of crass insensitivity, poor planning and a terrible way of handing a self-inflicted problem.