While it may sound surprising, it's completely legal for airlines to overbook flights, and, according to Jesse Neugarten, founder of Dollar Flight Club, they do so "all the time." He continues, telling Travel + Leisure, "Airlines are legally allowed to sell more tickets than there are seats on a plane because they know some people won't show up." Think of it as a numbers game based on historical data. "Most of the time, it works out, but every now and then, too many people show up, and someone has to get bumped."
Airlines often overbook flights, encouraging passengers to volunteer for later flights in exchange for compensation that can reach up to $4,000 and hotel stays.