I mean bombing government buildings (which is what landed Mandela in prison) is definitely what most people would consider terrorism, or treason, or similar things. Now you can argue that Mandela's actions were justified because Apartheid was evil (and I agree that it was evil) but that's entirely different than arguing that he was just a poor victim of the racist SA government who was imprisoned because he wanted to end Apartheid.
The problem is that people feel morally uncomfortable arguing that it's ok to bomb government buildings (and similar actions) when your cause is just, because that raises all sorts of other moral quandaries that most people don't want to (or refuse to) face. So they pretend like Mandela and his party were perfect angels practicing non-violent resistance like MLK so they can avoid the moral quandaries raised by suggesting that terrorism is ok for a just cause.
Showing restraint with atomic weapons is hardly a pass for lesser violence