A state broadcasting company not allowing negative coverage of the president is absolutely censorship. This sort of thing is what leads to a totalitarian state. If the people don't hear anything bad about their president, they will assume that he or she is good and that president will continue to be elected. This means that the president could be doing horrible things, but it would never be reported, so most people would never know. It's completely wrong.
Yes, restricting the coverage of the president to solely positive coverage by the South African Broadcasting Corporation is censorship. In a free-press situation, broadcasters would be allowed to say whatever they wish about the president. As soon as you restrict someone from saying anything in a public forum, it's censorship.
The journalist media of a country, especially part of the media calling itself the Broadcasting Corporation of the country, has a duty and responsibility to report the news and facts in an impartial fashion. By disallowing negative coverage of the President, the South African Broadcasting Corporation is derelicting this duty.
The South African Broadcasting Corporation is censoring their reporters by not allowing negative coverage of the country's president. This is a violation of freedoms of the press and speech. This also makes it difficult for the South African government to be transparent, and held accountable by the country's citizens. In short, this ban on negative coverage is a form of censorship.