[apparissus]

A non-zero percentage of passengers usually fail to show up for any given flight. It is more profitable for the airline to sell (e.g.) 110% of the seats available and get near 100% utilization out of the flight, while occasionally having to pay for a hotel room and a cheap voucher, as opposed to not overbooking.

When a business is acting in an abhorent, immoral, or unethical manner profit and greed are nearly always the fundamental reasons.

[chrisbennet]

How does this work? I mean, do the customers who fail to show, still pay for their flight?

[dagw]

do the customers who fail to show, still pay for their flight?

Depends on the ticket. For lower price tickets, generally speaking you don't get anything if you miss your flight. For Business and First class tickets you can generally re-book or get the ticket refunded if you miss your flight. There are also some insurance policies you can get which will refund at least most of your ticket if you miss your flight due to medical or family emergencies.