As a regular user of the line and who's family also use the line I can assure you that the on board staff rarely cruise the train looking for buyers, so passengers have to use barrier line staff to purchase at the city end, so why not at the country end also?
You and I appreciate the nicety of passing an open ticket office and heading for the exit, hood up, hands in pocket, which is a whole different animal than approaching RPI staff money in hand asking to pay. The crime is the intent. The OP had no intent - so no crime and no chance of a successful prosecution - but why cannot railway staff get this nicety into their heads and not create this PR disaster in the first place, or are these external staff - or New Northern which has been a PR disaster from day one!