I'll put it to you real simple:
Rwanda, Burundi and Uganda were run by criminals ignoring the law.
Apartheid was anti-democratic.
Do I think that some racists held white-run South Africa to I higher standard than other African countries? Sure.
Do I think that's what drove our foreign policy? No.
South Africa's strategic location and mineral wealth put it in the Soviets' crosshairs. To make South Africa safe for democracy, we first had to make South Africa democratic.
And yes, I also believe that we didn't intervene when Rwanda, Burundi, and Uganda killed tens of thousands or hundreds of thousands of their citizens because too many people think that African countries are "hell holes" where massacres like that are accepted.
Whatever you think have at it.