If you’d booked (H) then you would have paid more; they’re upset that you cost them the unrealized profit of P(H-X) - P(H), and also upset at the lost opportunity of selling a double-booked ticket to another passenger into your empty seat for an additional P(whatever), without having to refund you for your unused flight leg. Two profit opportunities lost for one service rendered is, to a corporation, roughly equivalent to kicking someone while they’re down, and they tend to react spitefully when these are exploited at scale.

Their claim of dishonesty is simply a guise for their desire to charge you the difference P(H-X) - P(H) when you deboard early, which under current law they are prohibited from doing. Your individual fuel costs are a negligible fraction of that amount, and while your absence is indeed pure profit for them in terms of the raw expense of providing services to you, they’re greedily maximizing all possible profits scenarios in order to claim injury and damages here.

To emphasize, I think this is bullshit whining about a predatory pricing practice being exploited in favor of consumers. I also predict that lawsuits will earn the travel industries a lot more strict pricing regulations if they continue to sue over this practice, so that consumers can expect the prices of individual legs to add up to the price of a multi-leg trip — whether on plane, train, or bus — and which would fully neutralize both their complaints and the exploit. (In review, it could also end up costing them the right to overbook if their legacy practices earn an appropriate degree of scrutiny.)