American Airlines, Delta Airlines, and Frontier Airlines are in hot water after violating transport rules. The Transportation Department outlined the violations in documents released to the public on Friday. The government fined American with $250,000 in fines, after concluding the airline didn't refund passengers timely in 2015. Delta was fined $200,000 for underreporting the number of missing luggage complaints. And Frontier is forced to pay the heftiest fine of all: $400,000! The airline involuntarily bumped passengers from overbooked flights.