Do you need a transit visa if you are in transiting China, but not entering, for more than 24 hrs.

Our friends have lay overs in Shanghai and Kunming to reach us in Chiang Mai, Thailand. The airline (China Eastern) refused them boarding passes because the total amount of time they would be in China (lay over time plus flight time) was 25 hours. Is this really true or was this a overzealous airline employee?