They are allowed to offer vouchers or free flights.
https://www.transportation.gov/airconsumer/fly-rights
Should the law be different. Maybe. But the law is the law.
They are allowed to offer vouchers or free flights.
https://www.transportation.gov/airconsumer/fly-rights
Should the law be different. Maybe. But the law is the law.
And even if they didn't, they could have drawn say 10 passengers instead of 4, and given them "a written statement describing their rights and explaining how the carrier decides who gets on an oversold flight and who doesn't" as mentioned in the page you linked, They might very well have found 4 people who would have refused a voucher but would have accepted a $800 check.
Or skip the stupid lottery and go with involuntary bumping of the 4 people who checked in last. Well, pretty much anything but what they did.
Not even "insists on it", they are required to pay cash or check, only, for IDB.
Their "dirty little secret" (which makes it sound 'naughty', as opposed to the more accurate 'illegal') is that for many years they've been offering vouchers in this situation.
And then there's this argument that the gate agent came on to the plane, and asked for volunteers, and then having no/not enough volunteers began involuntarily denying boarding, and the argument is that "having boarded, any removal of a passenger is involuntary, because by definition/law, voluntary denial doesn't involve demanding passengers give up seats after boarding or refusing to fly the aircraft".
Eh. No. We're talking about the point at which this became Involuntary.
The same site you link to has no mention of vouchers when it's involuntary, which is also the subject of my initial calculation and quote.
As this article, and many others have stated, this is but one of the issues with the handling of this situation, that United (though not the only airline by any means) has been quite happy to perpetuate the myth that airlines "only" have to offer vouchers. They can choose to offer vouchers for voluntary denial.
They are REQUIRED to ONLY offer check or cash for IDB situations.