It's diluting the message and giving a false impression - you either stand by the bylaw or you don't, Northern or any other TOC shouldn't give a conflicting message as they are doing - in two years of travel, I've nearly always bought a ticket on the train from Meadowhall or Sheffield after walking straight past ticket machines and the booking hall - only once has a guard ever told me that I should have a ticket before I board - 99% of the time they have freely sold me a ticket.
Without wanting to highjack this thread - I'm aware that it's best to keep threads asking for advice separate from discussions - I agree that Northern are far from consistent in their practice and this is bound to trip people up. In the North-East ticket machines have recently appeared on the Tyne Valley and Durham Coast Lines, but people still buy on the train in large numbers - partially, because one machine, on one platform, which only takes card, really shouldn't be considered adequate for some of theses stations. On these trains guards have long sold tickets on-board when departing from Newcastle, where there are clearly numerous opportunities to pay.
In many parts of Yorkshire/Lancashire they seem to have operated similarly for some time, and have only recently started enforcing the rules. That's their right but unless accompanied with information telling people that long standing practice is being revoked, it's at best morally dubious.
Again, though, I'd emphasise that in this case, there seems to be a high likelihood of successful bylaw prosecution if it went to court; but similarly a high likelihood of accepting an out of court offer of around the £80 mark if offered.