People are not thinking about Nelson Mandela’s death in prison, they are thinking of Stephen Biko’s death in prison. Anti-apartheid activist Biko was almost certainly killed by South African police torture in 1977, and it was a really big deal at the time and symbolic of the anti-apartheid struggle until apartheid ended. In 1980 Peter Gabriel released the song Biko, and it was a big hit and introduced Biko to the wider world. People just confused the two best known anti-apartheid activists.

Glad I got that sorted out for you!