I am sorry but I am seeing an exact analogy, other than your statement that services are somehow different from purchases. I am also depriving McDonald's from the added profit of selling me a coke and a burger separately for more than a meal, and I do have the premeditated true intent of not eating the fries, that I reveal by tossing them repeatedly every time I order a meal. Maybe it's harder to prove that I toss the fries every time, but according to your logic McDonald's in theory can collect the difference and threaten my credit too.
Again, nested depth limit but: I could have told the cashier "I only want a coke and a burger please" revealing my true intent. In that case, the cashier would have sold me what I wanted, for more money. Since I concealed my true intent from the cashier and said "I want a combo meal please" I deprived McDonalds of this profit.
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).
In this hypothetical, McDonald’s does not care what you do with the fries they sold you. Even if you told them that you were just going to throw them in the garbage, they’d still sell you the combo at the cheaper price.
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.
