Upvote:3
If I remember correctly, the white apartheid government was pretty clever at making the "it's us or the Communists" argument. That would have immediately fixated Reagan's opposition. Remember, at the time, there were a number of West-vs-Soviet flashpoints in Africa.
Now, I am not actually supportive of that argument, and certainly history hasn't turned out that way. But it's not uncommon for countries to avoid US condemnation by playing to US fears and perceptions.
Add to it that Reagan was not exactly at the forefront of progressive racial relations himself, "welfare queens" was one of his well-known campaign dog whistles. However, I am going to give him the benefit of the doubt here and assume that the main reason was the Communist angle, which is more in line with what he actually did while in power.
As far as the officially-given reason, "sanctions would hurt the people", that is an objection that is quite often voiced in similar cases. Both from those who would be at the receiving end of sanctions. And those that do not wish to impose sanctions, for whatever reason. Taking it at face value can be risky.
https://www.nytimes.com/2013/12/08/opinion/sunday/keller-nelson-mandela-communist.html
https://www.biography.com/news/nelson-mandela-terrorist-reagan-thatcher