Nobel Prize-winning economist Robert Shiller got a lesson last week in the economic principle of scarcity — at least when it comes to airline seats.

Shiller and his wife, Virginia, were kicked off an overbooked United Airlines flight to Aspen on Thursday. After no passengers took the airline's cash incentive to volunteer to give up their seats, the flight staff reportedly chose the Shillers to leave the plane because they had purchased their tickets via a travel web site. "Apparently our fare was the lowest," Virginia Shiller told Marketwatch.

Shiller told the website that United paid the couple $2,700,   but that didn't cover the alternative travel arrangements they  needed to make.

Airlines are required to compensate passengers up to  $1,300 for involuntarily denying them boarding, depending on how quickly the airlines can get the passengers to their destination.

Robert Shiller's Princeton colleague Nouriel Roubini tweeted about the incident, calling United Airlines "a bunch of idiots."

The practice of involuntarily kicking passengers off overbooked flights is not uncommon. In the first six months of this year, more than 13,300 passengers were involuntarily bumped from overbooked flights, and another 145,000 volunteered to leave an overbooked flight in exchange for compensation from the airline. Envoy Air and Sky West were the worst offenders.

Airlines have different methods for determining who gets kicked off of an overbooked flight, but in general elite travelers and those who check in earliest get priority. Flying during off-peak times will also reduce your chances of being booted, since those flights are less likely to be overbooked.

This story was originally published by  The Fiscal Times.