I was denied on a plane to Shenzhen because I didn’t have a visa.

I had a transfer in Shenzhen, a connecting flight with a six hour layover. The airline would not allow me to board without a proper Chinese visa, and I had to cancel my flight and buy a last minute flight home from another carrier. Everywhere online says there’s a 24 hour transit fee waiver for China. How did I get denied? And end up $900 out of pocket?