So preventing someone delayed through no fault of their own catching a train which doesn't actually leave for 5 mins and then charging for a new ticket is a good idea - sums up the 'GB railway attitude' nicely.And this is exactly why the policy is a good idea - to prevent latecomers from running onto the platform just as the train is departing, potentially causing a safety risk.
In the case quoted the passenger arrived on the platform just as the train was departing, whereas if the train had been displayed they would have arrived some mins before departure.