What probably happened is not that they failed to meet the visa exemption requirements otherwise they could not board the flight. They probably had a short layover and were not allowed out of the transit area.
I see this all the time on TripAdvisor where people come to complain (wrongly) that they were denied TWOV. Actually they were allowed to transit without visa, they just weren't allowed out of the airport. These days, based on recent reports, they do not let you out for a transit of less than 8 to 12 hours. There is no hard rule, depends on the mood of the staff. Having to pick up your luggage and check-in for the next flight is one valid reason to be allowed through.