Delta Air Lines is following the example set by United Airlines and saying it will drop an unpopular $200 fee on customers who change a ticket for travel within the United States. The moves come as airlines are desperately trying to lure people back to flying. Although air travel has recovered slightly since April, it is still down sharply. Normally in summer, 2 million or more people pass through security checkpoints at U.S. airports each day. That number hasn't been above 900,000 since the early days of the pandemic in mid-March.