Those aircraft do most of their work supporting the US State Department and other top officials of the US government. Very little to no support of USAF
Is there any reason for government officials to be in that area? How close is it to the embassy, to South African government offices, etc?
Would security for an aircraft be easier at that airport than at any others in the area? Often such visitors try to avoid big civilian airports like JNB
Back in the late 70s and early 80s - the USAF
flew a C-141 to Johannesburg twice weekly. It was part of a logistics flight bring mainly food items to Antigua, Ascension and Johannesburg. I was stationed on Antigua at the time. The flight did not land at JNB
but at another airport close by where it would RON and fly back to the US the next day.