There should be no PM restrictions for journeys such as Warrington to Birchwood, Wigan to Bolton or Heald Green to Wilmslow.
I once came home on a train from Manchester Airport to Wilmslow with a cheap day return but technically should have been excessed because the train left Manchester Airport after 4pm; what is silly about this is that anyone starting from Manchester would have been fine with a cheap day return because the train left there before 4pm. Anyway (unusually) I didn't think but didn't have my ticket checked anyway.
I suspect there's some not-so-hidden intimidation to scare people into buying the more expensive ticket "just in case" whereas, as
johntea says, if your plans change and you end up travelling in the peak it's only necessary to pay a simple excess, prosecution isn't allowed.
Northern was given a "choice" which came down to increasing car park charges or this, since I walk to the station I'm obviously unhappy about the choice that was made. But, really, it makes an already complex and generally unfathomable fares system even worse, and puts people off travelling by train even more I imagine. If you're lucky enough to be able to buy a ticket from a person they try and be helpful, as do I, because I ask for a cheap day return and say "I'm returning before 4pm" or whatever to speed up the process for everyone. Goodness only knows how it is with the dreadful ticket machines, I haven't tried to use them for ages.
Now we've had Covid-19 and a long-term reduction in people travelling at "peak" times, I think these sort of afternoon peak fares should be scrapped everywhere. Go back to "cheap day returns" after some time like 09:30, return at any time the same day, which was the system roughly since my youth, and generally well understood.
Similar restrictions apply from other places such as Birmingham New Street. That one's easy to circumvent, for those of us who understand the fares system, and that's also bad - people "in the know" can get round these annoying restrictions whereas "normal" people can't.
As a tentative first step in "fare simplification" it wouldn't be hard to scrap these annoying restrictions because we'd only be going back to where we were a few years ago. Let's do it!