Services can be different than goods in this regard. In the airline example, the seller prices flights differently for different destinations. When you indicate an intent to fly to city B by requesting a ticket with that destination, and the airline offers you a ticket whose destination is the one you asked for, their offer is made in reliance on your stated intent. So if you intentionally abandon your trip sooner, you reveal your true intent: to fly to city C. And if the price for flying to city C was higher, then you deprived the seller of a benefit they’d otherwise have had because they wouldn’t have sold you the ticket for the same price had you revealed your true intent at the start of the transaction.
Now if you tell me that I violated some written United contract, that I had to explicitly agree to when purchasing a multi-leg ticket, that's another matter. But I will still argue that that contract is not enforceable. And to my knowledge, they do not have a clearly visible contract with an opt in in this specific case.
My point is that the burden is on the airline to prevent such loopholes by revisiting their pricing schemes, rather than attempting to bully its passengers (also for Streisand effect reasons).
Law school is expensive, but I’d recommend attending if you’d like to better understand beyond what I’ve already said here.
United, however, does care how you use the ticket they issue. How can you tell? Because they would not have sold you the ticket at the discounted price had you told them your true intent (to fly to a city other than your ticketed destination). They offered you a particular fare because they relied on what you claimed. That’s the difference.
The key to analyzing these questions is, what would the other person have done had they known your true intent? If they’d have done something different, then reliance can be shown and that element can be satisfied.