Just to clarify a few things:
There are no signs at the station saying you have to buy a ticket before you board.
It is common for passengers to buy tickets on the train, and no warnings are given that you should not do this.
The exit station is gated so you cannot get out without having a ticket.
There was an anouncement on the train that particular day saying no tickets would be sold on that train.
The first point is unfortunately no excuse as it is a defence relying upon ignorance of the law, which is not generally a defence at all. As
Fare-Cop has said, there would be an excuse if the was a sign saying it is okay to purchase tickets on the train.
The second point is again not relevant - just because other people do it doesn't make it okay. I agree more warnings should be given though.
The third point - There is also no law that depends on your exit station, it is the origin station that counts (unless the exit station is your first opportunity to pay of course). I don't use Manchester Victoria much so can't comment - is it really gated 24/7? A lot of stations are only gated during certain times, so you still get chancers. The last point makes an observation of a strategy that is sometimes employed during revenue stings - unless there's RPIs on the train it is necessary to not sell tickets, so that passengers who broke the law can by caught by the appropriate, PACE-trained person.
I agree with you in one respect - it is very harsh treatment and I disagree with Northern's revenue protection strategy. They were actually denied a penalty fare scheme because their retail facilities didn't meet appropriate standards across the whole network, so instead they report people and issue these 'failure to pay' notices, which are out of court settlements. It does seem a bit bullying to some and many passengers mockingly refer to these notices as 'penalty fakes' due to the method in which they imitate the penalty fare system the previous franchisee wasn't allowed.
That said, I live in a town (Thirsk) with a station that is very similar, you have to climb steps, go down steps to platform 1, go back up steps and go down steps to platform 2 if you want to go North. The ticket office and card only TVM are located on platform 1. Most people seem to manage this no problem. You do get some people paying on the train and like in Adlington it's a bit sporadic as to whether they get a warning. You also get people who outright chance it... the number of times I've seen someone do the pocket dance before asking for a single to Northallerton, often to be given a stern warning but not really seeing the problem. Though I have seen two people reported for prosecution for just this. But overall most manage to buy a ticket. Suppose what I'm trying to say is that it's not exactly an onerous task and if it's required then it's what you do.