[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.