Your argument is a fallacy, because it is based on the presumption that a ticket from A to C gives the right to travel to B just because B is on the way.Courts can and do make changes to the law when they see a situation that defies reason or natural justice exactly like this. A ticket that was perfectly valid on the train suddenly becomes invalid by alighting after travelling fewer miles than the ticket allows, what nonsense.
Many on the forum criticize supermarket analogies, but perhaps they will suffer mine on this occasion: buying a 2-litre bottle of Coke does not entitle you to take a 6-pack of cans (and leave one behind in the supermarket), despite that they are the same* volume.
*yes there's a 1% difference, let's not get overly hung up