This is an entirely unsurprising 'development'. LNER have shown their disdain for walk-up passengers in many ways. From removing long-standing permitted routes (e.g. Edinburgh to London via the WCML) to eliminating return tickets, and now this.
Clearly they think they are an airline lumbered with serving such piddling intermediate stations as Stevenage, Peterborough, Doncaster and Berwick. Eminently the inconvenience caused to local passengers doesn't register with them.
What point is there in having a turn up and go frequency if you can't just jump on whichever train you like? Many of the ludicrous walk-up fares are supposedly justified by the high frequency on offer. Not much use if next 2 hours' (or the rest of the day's) trains are fully booked...
The only step remaining in their master plan to make all passengers buy Advance tickets is to eliminate walk-up tickets entirely.
I'm sure they'll soon make a start on that by restricting the ability of walk-up ticket holders to get reservations, e.g. reserving an ever increasing number of carriages and seats for Advance tickets. Trains will be "fully booked" even though Advance tickets are still available.