Even if it wasn't a passenger overbooking, it was overbooking in that they failed to properly account for the likelihood of having to ship crews around their network. If their occupancy ratio was lower, then there wouldn't have been a problem, they could have fit the crew in. If they reserved 4 seats for crew on every flight, again there wouldn't have been a problem. If they had even reserved 2 seats and run sufficie…
There's absolutely nothing wrong with this, they just didn't follow through with the correct way to free up seats, which is increasing the bid.
Some family cleared something like $13,000 in amex gift cards during the delta debacle last week by volunteering their seats multiple times. If you keep increasing the bid, you will find a price at which someone will give up their seat.