Overbooking is an 'accepted' airline practice but after Chicago maybe it need looking at. The airlines say they are taking into account estimated no-shows and may occasionally get it wrong. But if a ticket is paid for on a non-refundable basis it is no problem for the airline if that passenger does not show up, the airline has its money. The contract should go both ways and that seat should be guaranteed for passengers who do show up.
Who are these no-shows who will cause a loss of revenue for the airline? Are there any, or many?
Asking for volunteers to give up their guaranteed seat is acceptable and the offer might be attractive but if there are no volunteers those with a ticket for that flight board and are welcome.