Overbooking - that should not even be legal. Period!
All airlines overbook and have for decades. Based on history, there is a statistical percentage who cancel at the last minute. Without overbooking as an option, ticket prices would rise (someone if going to pay for the plane, the crew, and the fuel).
If overbooking were prohibited by law, then the playing field would be level and the airlines would marginally increase ticket prices to cover the few empty seats.
Here’s the deal, though: They were already on the plane and seated.