There is something in this whole UA "overbooked" flight that I don't understand, and forgive me in advance for my appalling ignorance.
Airlines claim they have to use the practice of overbooking because in the event of a "no show" the empty seat can not be sold and that means loss of money.
But my logic tells me that even when a passenger is a "no show" his or her ticket is still payed for, since airline tickets are generally payed for in advance, and can not be refunded AFTER the plane took off. So why does the airline care if the seat is empty?
Where am I wrong here?

I.M.