In a comment to this post, Joy asked why it was that American TV shows were shown in South Africa during the apartheid era... I didn't know, but it seems there are two main reasons:
1. The US actors and related unions/guilds did not declare a ban on their works being exported to South Africa. (If you find information to the contrary, let me know...) The British Equity actors union did enforce a ban. (British and American actors and musicians resolved not to perform in South Africa, although some infamous breaches of this occurred...)
In 1976, Equity in Britain decided to introduce a policy of refusing permission to sell programmes featuring its members to South African television. The Council of Equity also reaffirmed its policy to advise its members not to work in South Africa.
In October 1981, the board of the Associated Actors and Artists of America - an umbrella organisation of all major actors’ unions with a total membership of over 240,000 actors - took a unanimous decision that its members should not perform in South Africa.
2. From this wikipedia article:
The availability of US programming was partly the result of a co-operative venture with Universal Studios in 1980 where an episode of Knight Rider was filmed in the Namib desert in South West Africa (today Namibia), and local acting talent was involved in the filming. As a direct consequence, the SABC received the right to broadcast in American programming syndicated from Universal Studios/MCA, and through them purchased material from other studios.