Earlier quoted context omitted.
Then the airline needs to leave a seat unsold to allow them to do this. I hope they get sued within a cent of their ability to continue operating.
So, let's just say for a moment that this applied to 100 flights per day, and meant that United foregoes $1,000 in revenue per seat that they don't sell. They would be giving up $100,000 per day, or $1 million every 10 days, or roughly $35 million per year, by leaving a first-class seat empty on every flight. Weighing that against the risk of pissing off one or two first-class passengers a month for those times when…
I didn't say it did. They have to tell their top paying customer "no" or charge enough to make this possible. Humanity/morality should come in to the equation at some point too.
An alternative of course is to run an auction to be bought out of the seat, but they can't absolutely guarantee a seat that way. If they want to guarantee a seat, they must keep one; nothing else is a guarantee of a seat, only a promise to try and give one.