Yes, I'd be amazed if there wasn't a similar approach to how the barriers were brought in at Leeds and most other places, accepting any old ticket for a while, then only causing problems when the protocols were activated. I'm sure the complaints will start racking up once staff are told to start enforcing the letter of the law, only by then I think Northern will hope the publicity will have died down.
The big issue as I see it IS the letter of the law: it's too fuzzy, too much open to misinterpretation by those (most likely the contractors) who aren't aware of all the possible reasons you wouldn't be able to purchase your ticket despite having a card, let alone be familiar with them. Rovers, seasons, Privs, boundary, split, off network through tickets to heritage lines, I'm sure they'll all have been mentioned: it can be difficult enough getting them from normal sources sometimes. I can fully imagine people being told by contractors they have to buy a week's season to Newcastle ("Where are you going?") with cash ("It says 'CASH ONLY'") instead of a Freedom of the North-East for example! That might be at the extreme end of the scale, true, but as many people on here will be aware they are notoriously inflexible, like trying to negotiate with a pair of rhinos usually!
Just reading it again and the wording seems to suggest that you'd be able to choose not to pay the member of staff on the train and instead pay at your destination! Being slightly pedantic but is even the 'owe' bit really correct if the bearer wishes to buy a rover or season of some description: you can't owe for travel you have yet to undertake for which you've not yet purchased a ticket can you? There's just too much that's not clear about it all, and F in plain English!
Rather than state 'by cash only', I suspect 'on demand' (with the usual Electron exception) would be more suitable.