A suprisingly large number of people believe you can buy tickets on trains even if the office is open. Literature from TOC's often does nothing to sort this issue and neither does the fact I can't remember the last time I saw a train manager inform a passenger they were in the wrong. I am often on XC and FGW services from large stations with long opening hours and the train managers happily sell Anytime tickets onboard, further perpetuating the belief among many of the travelling public that this is correct and normal.
I will sell a ticket when I find somebody without one, as that is my only realistic option - I can't issue a PF, I am not going to delay the train at Reading to chuck the passenger off (and then read innumerable complaints and posts on this forum about "poor customer services"), and a TIR requires the time-consuming (and dangerous) process of establishing their name and address.
I will ALWAYS point out to people the requirement to buy before boarding, and by not doing so, they have committed a bylaw offence and are now liable for a Bylaw prosecution. As a rule of thumb, out of every ten, five will respond with indignation that I dared question them (and three of them will say"But it's your job to come through and sell me a ticket), three will listen politely but have obviously heard it before, two will thank me and say they didn't realise, and one will turn out not to have any means of payment.
The most common tickets are Pad-Rdg (trains every 6-10 minutes maximum), the most common excuse "I was running for the train", despite the frequency, and in some cases, I have noticed the individuals having been on the train 20 minutes before departure (I can SMELL the ones without tickets when I walk through).
Paddington station has posters up on every platform, and automated announcements that there is a "Buy before you board" policy.
At this stage, the only thing that will prevent it is either putting ticket barriers on each platform (as you only find the people who didn't have time to buy a ticket on those platforms with no barriers) or putting RPI's onto the HST's and issuing PF's / Anytime tickets to all for a few months - but FGW does not want the complaints this would create.
Nobody thinks the railway is free. However, as I have argued many times on this forum, a significant number of people don't buy tickets because they are able to get onto the train without one, and it is not that they think they can buy on board, rather that they think there is a chance they won't have to buy one at all.