Okay, I was under the impression that most regular airline tickets are still mostly refundable and / or easily canceled. I also didn't know that the money they offer is a voucher; European airlines hand out actual money, usually the same day.
What I was mostly defending was the strategy of airlines responding to regular no-shows and how I think it's not surprising to expect a large, profit-hungry airline to overbook just to make sure planes are full. It's the 'ZOMG overbooking is evil' responses that I thought were unfounded and not the issue here. They should've just offered more or rerouted the United crew.