Just tell them that you are required to fly to countries where an entry stamp in your passport from an immigration officer of one country might jeopardise your safety and security. Such jeopardisation might involve the local South African consular office which would involve unnecessary expense for the South African government.
Countries to use, as an example, for this claim would be Israel and Syria, Pakistan and India, North Korea and the United States, Britain and the rest of the world.
Best of Turkish luck to you by the way.