That's circular reasoning!Even if you are able to rock up and buy their own tickets at a regular ticket office or Ticket Machine?
The examples you cite are basically charter services, albeit regularly operating. Not really intended for "normal" journeys, hence lack of inclusion in NRE.
The only reason you'll be able to buy ECTL tickets from the ticket office (and the minority of TVMs that sell Advances) is because the services are in the National Rail timetable.
If they weren't in the National Rail timetable you would have to buy them in the same way as a charter service.
I think there is a lot of scope for confusion, particularly on the shorter flows (e.g. Morpeth to Newcastle), if they get to pick and choose which bits of the National Rail system they participate in. "Any Permitted" wouldn't really mean "Any Permitted".