USAID's work in South Africa was an attempt to paper over the obviously communist and worker led anti-apartheid movement and take undue credit for ending apartheid. If you look it up, defenders of USAID love to say that they "laid the groundwork for ending apartheid" and "were an important part of ending apartheid" which is obviously just racist nonsense. No amount of US support or opposition would have been needed to ensure success or hinder the movement, it was an African movement by and for Africans whose time had come.
As you can tell, their role in being "anti-apartheid" was in line with their soft power goals and typical scope of work, but in this case since it was clear that they weren't going to get their people in charge, they had to look like they were in the right side in order to legitimate themselves to the anti-apartheid movement that would be taking power.