On Saturday me and my mum went to Weymouth. We caught the 09:17 loco hauled service from Keynsham which was very enjoyable but had no FGW▸ staff onboard to collect fares. so, for the majority a free journey.
My Emphasis in your quote.
My understanding of this service is that it runs from Bristol Temple Meads, which has ticket sales and access control to the platforms, and calls at a series of stations in a "Penalty Fare" area, where you are required to buy your ticket before you board the train. There's a machine at Keynsham, a ticket office at Bath and barrier control, ticket offices at Bradford-on-Avon, Trowbridge and Westbury ... all of which should (?) be open at the time the train left.
Now ... it's possible that a proportion of passengers might try to get on the train via various wheezes through which they don't pay the fare / proper fare and I would agree with you if you had suggested that those passenger would in the circumstances have got away without paying. But I would have been utterly amazed if that was
the majority, or even a significant proportion, until you told us. Do you know something that I don't, perhaps?