So at the end of the day, I think it therefore boils down to the difference to United being too abstract and intangible to understand, and therefore the provisions of the contract are somehow unreasonable and unenforceable. If I understand correctly, the argument against fraud is something like "United's contract makes no sense, therefore it's not enforceable, therefore violating it is perfectly legal".

Obviously, you can leave the airport on a connection if you want to (both physically and legally - I have entered and left Australia while transferring there) - but the intent of the contract is to bind you to not "access" the transfer point in the meaningful way of having provided transportation there, in whatever sense that means, but in practice by boarding your next flight. The abstractness of such a restriction versus the concrete presence of faregates at a train station is an interesting distinction in the reaction it provokes. Indeed, many railways operate more similar to airlines (no faregates, proof-of-payment) but they generally don't even attempt to collect a higher fare for a shorter journey because it is too hard to enforce. The other difference with the BART example, of course, is that the tickets to SFO and Millbrae are always that price with no variance, eliminating another source of confusion and better quantifying the difference.