The concept of the NRCoT pre-dates devolution, but trying to maintain one document valid across different countries with their own legal systems is getting increasingly complex, so may need looking at.
Because Scotland is a devolved nation with it's own laws, parliament, and rail fares. It shouldn't automatically follow that what they do is what England and Wales does.
But GB has one rail network and a lot of journeys are cross-border.
Don't get me wrong - I'm not hostile to either devolution or full independence. In fact, I'm rather more open to it than the SNP seems to be, at times. I don't, for example, think London should have a veto.
But even if there were two separate countries it would still be the case that, as with the island of Ireland, there would be one rail network. It's hard enough for people to know and adhere to the rules when you have one set of T&Cs, let alone if you start being different for different's sake.
The rules are clear everywhere else. Buy before you board if there are facilities at the station where you board to buy the ticket you require, unless those facilities don't accept your only available form of payment. If you don't, there's a range of options available to TOCs to deal with the fact you've travelled without a ticket, one of which isn't to make up a hitherto non-existent fare outside of the simplified Anytime / Off-peak / Advance range.