In regards to nazism, its a defeated dead ideology that is considered more or less a joke by everyone, outside of a few diehard skinheads and those drawing edgy stuff because they find it funny or like the outfits/style it had (japan has a whole thing about this as well where people draw characters like that not because of the ideology but because of the style).
In either case, by allowing the free posting of either here encourages dialogue/convos about such topics, and it being out in the public view makes people keep it in view and in mind.
Our whole fandom is built on a crazy mix of stuff like this, stuff that was rejected by the world at whole, and yet somehow this fandom grew and grew and achieved some hilarious and at times insane things. By starting this forced segregation/banning of particular things, it will only fragment the fandom more.
If people are really that turned off by something, the block lists are perfect for that so that they never have to engage/see it, but is is a choice they make for themselves.