Although missing legs can cause problems for travelers, it's not actually illegal as such. After all, all you are doing is buying a product from an airline, and failing to redeem part of it. So legality isn't the issue. Unfortunately, this is an area where the "conditions of carriage" come into their own.
Is it OK to skip a leg of a flight?
Re: Can I intentionally miss 2nd leg of connecting flight? Your ticket was priced on the basis of the legs and order chosen. This forms your contract with the airline, so if you skip a leg, the rest will be voided.
Can you skip a leg of a connecting flight?
Can you only take one leg of a connecting flight?
No, unfortunately you cannot skip the first leg of your flight and start your journey from the second airport. Airlines have strict policies in place to ensure that all passengers board their flights at their designated departure airports.
Do airlines like Skiplagged?
Airlines despise the practice. Dengler laid out a couple of reasons why. “Airlines do not like skiplagging because it costs them money. Flights with connections are generally cheaper than nonstop flights because airlines have a lower price ceiling for them,” he said.
Can you take the second part of a connecting flight?
If you miss the first flight without telling the airline and just show up for the second leg, the ticket will certainly be cancelled. If you call them and tell them you no longer need the first leg, they will change the ticket for you to remove it, but there probably will be some sort of fee.