A bunch of reasons: anarchists has pretty loose moderation, and a lot of people think that there should be no moderation or banning at all. (In free_palestine, neo-Nazism will pretty much guarantee an insta-ban.) Many anarchists don't understand what neo-Nazis are or the methods they use. And yeah, in general I think the anarchist movement is more tolerant of wingnuttery in the name of inclusiveness -- which is actually not very inclusive, since the presence of wingnuts alienates sane people.

I get your second point