I don't really get how many people claim they don't know that they must buy a ticket with all the publicity about.
Quite simple. The result of the almost-daily experience of seeing people buy tickets from the guard on the train means the "must" gets downgraded in your mind to a "should".
If it were possible to purchase a full range of tickets at any station in the few minutes before the train arrives, using any payment method, and be sure you're buying the right ticket(s), it would be so much easier for people to understand "must".