In this case, the low fare due suggests a short or local journey. Should one really have to buy a smart phone, obtain a credit or debit card, and use an app to make a local rail journey ? Talk about making public transport less accessible and more complex to use !
And as for the suggestion that the customer should have arrived earlier, no one has stated just HOW early is expected.
The vast complexity of modern railway ticketing, and the apparent slowness of modern ticket issuing equipment are backward steps, it was not passengers who asked for greater complexity and slower machines, so why should we have to pay for it.
As others have posted, northern have a clear incentive to make it hard to buy a ticket, in order to maximise profits from the spot "fines" the imposition of which would be called extortion in any other business.
Drifting only a little a little O/T, the law has been recently changed to curtail some of the dodgy tactics of money lenders and vendors of goods on credit.
It would not surprise me if the rail industry is next.