If someone buys a ticket and does not show up for a flight then usually they don't get a refund/credit (or a crappy one: I'm looking at you United).
But now the airline has an empty seat. The best thing to do is fill it. But you can't fill it the last moment so you need to overbook. But if you overbook too much then you need to start bumping people off. So the airline gets to keep all the revenue from the overbooking as well as the fees for the refund which can be substantial. It's a bit like double dipping.
I remember as a kid flying "standby". You got a cheap ticket but had to sit around until a seat opened up. What killed standby tickets? Post-9/11 security hell?
People should support airplanes flying full.
Ok, so environmentalism doesn't matter if it's inconvenient.
That's pretty far from greedy.
> This per-passenger profit is roughly double what airlines around the world make per passenger, according to the International Air Transport Association