The question about South Africans, or any other nationality, might not be a rule as such. The CAAC does a very thorough background check, including calling your home country's civil authority. If they have 20 boxes to fill, and they can only fill 19, you don't pass. Maybe South Africa, for some reason, won't satisfy their security requirements.
They got burned in the past by bogus credentials, and they are hard over the other way now. Unfortunately in filtering out the chaff, they might be filtering out some real targets as well. And there is probably nothing that can be done about it. TIC (This IS China)
Good luck, however