Originally Posted by PHL
Illegal is not the right word, but it's against FAA regs for a scheduled operation to cancel a flight solely for low loads. But, like others have said, it's usually not an option anyway since the equipment often needs to be in the next city to service a new flight.
Probably more like DOT regulations.
Flights with low loads may be more prone to cancellation due to equipment failure/mechanical problems. I've seen it several times (and including when I worked for a major carrier). They have to cancel a flight, they don't have a choice, due to maintenance on a plane. Rather than cancel the full flights on the aircraft's scheduled round trip, they cancel a lightly loaded trip to another destination instead by swapping it's plane out with the MX aircraft.