I disagree on this. If a 'regular passenger' buys a ticket CPH-JFK they are not expecting they have two separate tickets CPH-FRA and FRA-JFK. They search as a single ticket and that's what they are given. Same with a QF flight MEL-LAX... I don't think any member of the public thinks they have two completely separate tickets?
No, but they think they've paid for a trip that goes A-B-C and if they happen to get off at B, why should the airline care (indeed, it should be happy for the lower opex). Similarly, they would expect to be able to miss getting on at A and still be able to fly B-C for the same reason (eg: if they run late and miss the first leg of a trip but realise they can still get to the transfer point before the second leg leaves using another airline - yes, someone has told me this happened to them and were both surprised and aggravated when they tried to check-in for the second leg).
There's an obvious variant of this that more "regular" people will be familiar with, and that is buying a return ticket and for whatever reason not getting on the first flight (plenty of legitimate reasons this might happen, from simply running late to a medical emergency).
A "regular passenger" would not expect this to cancel the return leg. I can say this with some confidence having (a) been such a "regular passenger" (though still having taken dozens of flights) myself many moons ago and getting caught out by it (quite seriously both in terms of inconvenience and $$$ - not only missed a mate's birthday party but it cost me well over a thousand bucks all up to recover from, when that was a lot of money to me) and (b) talked to dozens of people over the years who were either caught out themselves, or who were extremely surprised to think it would be a problem.
A "regular passenger" who discovered (or heard about) hidden city ticketing would just think they were being clever and getting a bargain.
I would be very surprised to see a court rule on the side of the airline, even in this day and age. The rules that it breaks are buried in the ticket conditions that nobody (normal) reads. I would expect it to be treated the same way any other sort of buried-in-the-fine-print-condition is (eg: software EULAs), which would be some sort of reasonable person test. Ie: would a reasonable person who bought a ticket that traversed locations A-B-C expect to be able to get off at B without any consequences, just like they can on trains, buses, taxis, tuk-tuks, or whatever. Answer: of course.