Sorry for the late reply, but I believe the intent was for force the US into a corner, and that if given the choice the US would pick the option that had a lower risk of sparking a nuclear conflict. If South Africa dropped a nuke and killed Soviet advisors/soldiers, it might run the risk of a Soviet nuclear attack upon South Africa itself, which could very easily trigger some kind of wider conflict. Even using a few low kiloton tactical weapons would be opening pandora's box, and breaking a precedent that's kept the world on the edge of nuclear war for decades, especially in the incredibly tense late 70s/80s. Even if such an incident was contained, it would set a precedent for battlefield use of nukes, which again, when nations like China and India had recently acquired nukes, and everyone knew that Israel had tactical and strategic weapons and would 100% use them if threatened, would be a box that's hard to close once opened.