Read an article in the paper yesterday about skiplagging, the practice of using only the first leg of a multi-leg flight. Apparently for some routes, you can buy a cheaper ticket from Chicago to San Fran with a stop in Denver, than a direct flight from Chicago to Denver. So people who want to go to Denver buy the ticket to SF, and simply get off in Denver, causing the seat on the Denver to SF leg to go unused. The linked article explains it pretty well.
The airlines strongly oppose this practice, and may penalize people they catch doing it by cancelling their frequent flier miles, future flights, or even legal action. It is not illegal, but it violates the airlines contract of carriage. Apparently the prohibit use of only the 2d (or 3d) leg if you did not take the 1st.
The practice never occurred to me, but I have a hard time thinking of skiplaggers as doing anything wrong. They paid for 2 flights, but only wanted to use one of them. If there is any loss, then it results from the airline’s pricing decision.
If I pay for a multi-course meal, but decide I’m full before dessert comes, the restaurant does not hold me down and force dessert down my throat.