The U.S. DOT O&D survey requires that U.S. airlines submit all of their tickets whose last digit is a 0.
Airline employees flying for free are not issued actual tickets, so they would not be reported. These non-revs include deadheading flight crew members or airline employees taking advantage of their flight benefits.
On the other hand, passengers flying on frequent flyer tickets are issued tickets, so these passengers are reported in the dataset. Diio Mi shows these passengers in various reports like the U.S. DOT O&D --> Dynamic Table and the Tools --> Airline Performance as "% Free". The low fares that these passengers pay are included in the average fares for these routes.