@njzk2 When you're wrong, why does anyone owe you proof? They don't. Even moreso when your motivation is to indulge the wrong view for selfish reasons, e.g. cheating/stealing yet claiming not to be doing so. (If you doubt what I'm saying, try arguing with a climate change denier. You'll get it PDQ.) When you take something against another's will, the burden is on YOU to prove it's not stealing.

@Therac "This is not theft of services, because the carriage contract does not oblige the customer to take all of the flights" Does the carriage contract oblige them to pay for the value they took?

Booking a ticket without intent to fly any or all parts of the itinerary is not deception. It can make the contract invalid, but it likely does not rise to deception under law, as no airlines I've checked so far ask the client to affirm their intention to travel, even if they "prohibit" avoidance of fares of the real itinerary in CoC. They may be able to ask for civil compensation, but the rules on the compensation may need to be sufficiently clear (how Lufthansa lost the case in Berlin)

@Harper-ReinstateMonica No, it does not. The carriage contract - at least any common one - doesn't include a provision to oblige the customer to pay for the value of any other flight.

@Johns-305 Back in the day, the double feature would likely be priced lower. That's the fun of contract law: once two parties agree on their rights and obligations, it doesn't matter it one obtains more value than the other had expected it to. A ticket gives you the right to visit the movie theater - not an obligation to stay for the B-movie (or the flight leg you don't need).

It's entirely possible to create contracts that incur penalties for not enjoying your rights to the fullest. For instance, gym memberships that charge less if you actually attend. Restaurants where you have to pay extra if you don't finish your meal. And in B2B, it's common practice to include penalties for both under- and over-delivering. It's easy. The contract just has to say it explicitly. And common carriage contracts don't.

@Harper-ReinstateMonica Update: actually, some (not all) airlines have included such a clause. Courts have found such contracts unenforceable. Lufthansa's civil suit was the first to survive dismissal (meaning the court would even hear it), and it has been dropped since.