In a statement to police, Robson, who had been driving tour buses for 28 years, said he had not seen Mr Oh, and had been completely unaware of what had happened until he stopped his vehicle and alighted from it.
One of the defendant's passengers told the court that the bus was travelling very slowly, other evidence suggesting a speed of no more than 5km/h, or perhaps walking pace.
Judge Field had observed from the video record that sandboarders achieved a quite significant speed as they descended the dune, and for a time over the flat surface at the bottom, "so it seems clear that Mr Robson was driving the bus at a very slow pace."
Another passenger said Robson had stopped for a woman who was walking ahead of the bus, tooting at her. She saw a sandboard descending at a speed that appeared to be too fast to stop; the descent of the dune had taken only a few seconds.
Another tour bus driver told the court that he had been "setting up" a sandboarder when he heard Mr Oh's board hit the sand behind him. He put his hand out to stop him and told him not to go. He thought Mr Oh had understood, but clearly he had not.
He confirmed that Robson's bus had been travelling "really slowly".
Judge Field said there was no issue of excessive speed, "or anything of that sort," on the defendant's part, likening the situation to a pedestrian moving rapidly out in front of a vehicle that was otherwise being driven properly.
"He was proceeding cautiously at about walking pace along the bottom of the dunes, it would seem some 14 metres from the base of the dune," he said.
"Other sandboarders had stopped earlier than Mr Oh, probably because they were lighter and Mr Oh had more momentum."
He did not agree with the prosecution that if Robson's bus had been stationary at the time of the impact Mr Oh's death would have been unlikely.
"It seems to me that there can be no criticism of Mr Robson's driving in these circumstances," he said.
"There was no contributory negligence here, and it has not been suggested that that formed part of the scenario."
He was not satisfied that the charge had been substantiated by any evidence that the court could accept, and it was dismissed.
Judge Field expressed his sympathy to Mr Oh's family, who had suffered a tragic loss in shocking circumstances.