It depends on why people want to read it, just as why anyone might want to read Chairman Mao Tse-Tung's Little Red Book.
We don't need read them to know how awful these rulers' regimes were, and it would be wrong to read such books to inspire copying them.
However, to understand any such leader's motives beyond simple "news-reporting" history of actions, does need knowing what these men actually said for themselves.
It's no different from anything else except in scale. You might for example support one political party over another in your own country's' election but you cannot do so sensibly if you do not comprehend your opponents' views - even if you still disagree with them. You cannot understand a war in a distant land, or a bitter industrial dispute at home, unless you listen to, or read, facts and opinions from both sides. And you need try to do so as dispassionately as possible before making any judgment.
So although we know what those regimes did, and however nasty the background dogma espoused by Mein Kampf and The Little Red Book, censoring or forbidding them would be just as wrong as their authors' similar destruction of their own opponents' words.
