There's a key difference in this case though, namely that it's not a case of a government trying to look forward and anticipate whether an ideology will cause damage, but instead a government looking back on an ideology that actually did cause damage.

Nazism cost millions of Germans their lives, led to one of the biggest campaigns of genocide in history and wrecked the economy, social structures and world perception of Germany for decades. So the question about it isn't so much "is this thing dangerous?" as "how can we best keep this dangerous thing from getting loose again?"

(Which isn't to say that banning it is necessarily the right way to do that; many have argued that banning it just gives it an air of uniqueness that makes it more attractive, rather than less. But nobody making that argument would go from there to say that Nazism is somehow morally neutral.)