I'm not sure why the RMT are complaining though, it doesn't affect their members in any way.
This perhaps misunderstands how the trade union movement, at its best, works collegiately and tries to advocate for the interests of all workers. Passengers are, overwhelmingly, workers. It is also possible that, unlike the senior management of the companies that employ their members, they can see further than the end of their nose and they realise the implications of a "charge what you like" approach being permitted i.e. passengers will be driven away from the railway. I suspect they want capacity shortfalls addressed by improving and expanding the railway instead of suppressing demand by overcharging.
While this particular instance is hardly significant, that is of course how you introduce a culture of overcutting regulated Anytime fares - you start by doing it by a small amount and you do it at major events where the case for suppressing demand is stongest (not that this level of overcharging is going to do that: people bought their tickets for this event months ago and they have to get there).
No; they are not allowed to do this.
However, we do not have a regulator, ombudsman or passenger watchdog who actually has the intelligence, desire, knowledge and clout to actually do anything!
Indeed. If they can get away with it and keep getting away with it, we'll then start seeing policy papers coming out of DfT saying the current rules are "outmoded" or "unworkable" and proposing that the last vestiges of fare regulation are swept away.
Passengers intending to make a journey are entitled to look up an Anytime fare and think "Okay, maybe I'll be able to work out a cheaper way to do this nearer the time when my plans are firmer, but at least I know that's the ceiling. That's the most I'm going to have to pay". This is the kind of information people need to know for budgeting. Not so much, of course, for a short journey like this that only costs a few quid, but it's the principle. This could apply to any journey.
I'm not commenting about whether or not the right to buy walk-up fares exists today in law or in regulations, but it has been the custom and practice throughout my lifetime that, as a passenger, you can rely on the relevant published walk-up fare as being the most you'll have to pay. It would be massively detrimental to passengers if that were to change.