The Department of Transportation is investigating United Airlines after the carrier sold tickets Sunday to and from Hong Kong for only four frequent-flier miles plus taxes, then announced it wouldn't honor those reservations. It may be the first major test of a new pro-consumer ticket-buying regulation.

In January, the DOT prohibited airlines from increasing the price of air transportation after a purchase was made. The concern originated with airlines imposing new baggage fees retroactively on consumers already ticketed....