India, Oct. 8 -- An unoccupied seat on a plane does not automatically grant permission to a passenger to occupy it. Some travelers on United Airlines learned that the hard way last month when a social media post about a sudden $86 surcharge went viral, catching attention online.

"@united flight to our mission trip to Honduras from Houston. 3 hour duration. Most of the plane is empty. The attendant told us that they won't let anyone change seats for less than $86/each even though the plane is nearly empty," one passenger wrote, sharing a photograph of empty rows in front of her. "What would you do? Pay the money or stay out in your economy seat?" she asked.

According to USA Today, it is not just about money. Airlines have legitimate reas...