Or they can just take the hit. In this case, they decided to comp four people so they didn't have to cancel a plane load. Seems like good economics, but no one wanted to give up their seats. I mean why would you. It's bad enough you have to get molested by the TSA once. Twice? Fuck that. I want to go home. No I don't want a voucher for $200 of my next flight with the same shitty airline that will expire before I can use it.
They should have just realized they can get a crew to that plane and canceled that flight; take the hit. Those passengers haven't boarded yet and would be easier to deal with.
You are legally entitled to cash compensation over an airline voucher -- you just have to ask for it and a lot of people either don't know that or just don't do it.
Also the rebooking is not necessarily on the same airline if they don't have a similar flight available that day but others do.
Yeah, good point that cash must be explicitly requested in advance for voluntary denied boarding vs involuntary. The Points Guy has a pretty good guide for VDB.
By cancelling they could be cancelling all subsequent flights from that plane for that day. If I had the choice between kicking 4 people off a plane and cancelling potentially multiple flights I don't get how cancelling the flights would be better? Getting pilots and planes where they need to go after the planning gets messed up is incredibly expensive and also screws a bunch of passengers
They should have just realized they can get a crew to that plane and canceled that flight; take the hit. Those passengers haven't boarded yet and would be easier to deal with.