It’s a fucking crazy system that airlines are allowed to overbook flights so they frequently find themselves with too many passengers and have to offer incentives for people to take delays. I understand why it happens - that enough people miss flights that they’d be flying with empty seats, which is less cost effective, so they’d just charge more and pass than on to the customers - but since I’m going to be ripped off by an airline regardless, you may as guarantee me a seat tbh.
Saw a situation very like this last time I was flying internally in the US, can’t remember the exact ins and outs but they they let a guy board the plane who was borderline and just after he sat down, someone came and apologised and said he’d have to deplane - he refused, said he already had his seat now and wouldn’t be moving. Went on for about 15 minutes, escalating levels of staff coming to tell him to leave and him refusing, eventually security came and he went with them relatively calmly. But he had every right to be furious.