From a strictly moral perspective, there is no principled argument that it was unjust to engage the apartheid regime, yet just to engage North Korea’s. For all its evils, apartheid did not consign hundreds of thousands of people to political prison camps, or starve a million or two to death. For that matter, North Korea is every bit as racist as South Africa ever was, and even manages to have its own system for imprisoning a majority of its people in economic injustice, poverty, and hunger from the cradle to the grave. (It also compounds the sins of mass starvation, domestic terror, arbitrary execution, and democide with both sexism and homophobia.) Despite the world’s unfocused outrage, North Korea denies the very commission of its crimes against humanity, a nearly sure sign that it means to go on perpetuating them, as long as we allow it to.
It was deemed moral to not engage with South Africa while it had apartheid. People can punish little old ladies for not offering flowers for a gay wedding yet many are hesitant to hit North Korea (and its backer, China) where it lives.