No-show clauses in airline ticket contracts have been considered by the Tribunal Supremo (the Supreme Court) of Spain, in Judgement 631/2018; see section 6. Importantly, they applied EU legislation, the Unfair Terms in Consumer Contracts Directive 93/13/EEC, which means that the reasoning may be applicable elsewhere in the EU and in the UK, although it is not binding on those foreign courts. That is also important because it cuts across how that contract law operates in different member states - those countries could have very different bases for analyzing contracts, but the prohibition of unfair terms should work in the same way in the end.
In the case, Iberia had a contractual term (with my emphasis):
Dependiendo del tipo de tarifa, clase de servicio, estancia en destino, oferta, etc., puede realizar su reserva para vuelos de ida o ida o vuelta. Independientemente de la tarifa aplicada, si alguno de los trayectos comprados no se usa, automáticamente se cancelarán los trayectos restantes comprendidos en el mismo billete.
This would allow the airline to cancel any remaining legs of the journey in the event of a no-show. For example, if you bought an A-B-C return, intending to use only the B-C part, then you might find on showing up at B that the airline had cancelled your entire ticket because you had missed the A-B flight. A consumer rights organization argued, and a lower court agreed, that the Unfair Terms directive should apply. They said the clause created an imbalance in the mutual obligations between the contracting parties: it means the flyer is obliged rather than merely entitled to take all of the flights, which is contrary to a good-faith understanding of how tickets work. The Tribunal Supremo agreed, rejecting a counter-argument from the airline that pricing matters are excluded from the scope of the directive. They also said that if the airline was willing to take a passenger from A to B to C at a certain price, then it should also be willing to do less work for the same amount of money.
In the decision, the Tribunal Supremo was equivocal about people who deliberately use no-show strategies. The rejection of the clause was mainly because it covered people who had changes of plan due to unforeseen events. In paragraph 8 of section 6, the court conceded that the airline might be viewed as suffering harm if people were following the "exceptional" practice of hidden-city ticketing, where the total price was lower than that of any individual part of the journey and the airline could not resell the unused legs in time. If the consumer is deliberately taking advantage of a pricing loophole, then it could also be argued that because they are not acting in good faith, they should not get the benefit of consumer protection law. The court did not consider this matter in much detail, since the term was already deemed unfair in the case of consumers affected by unforeseen circumstances.
So there is some potential room for terms and conditions that are more nuanced than Iberia's. In practice, it appears that Spanish carriers at least have not tried to push back in that way.
There is a potential revision to the Air Passenger Rights Regulation 261/2004/EC which would allow consumers to no-show without having other parts of their journey canceled. But this has been under discussion at least since 2013 and has not progressed very far. If enacted, it would apply definitively across the EU, as an even more specific lex specialis for the scenario in question.