You could replace nazis with any group of people with a specific message that they want to spam all day, every day and it would still be a horrible decision to let them roam free in every public forum. Whichever the group is, not ALL forums on the internet should be about the grievances of that one group. Though, it just happens that nazis are the ones who seem to be the quickest and most common group to take over forums. This is because they need an audience for their crap, and on every forum either the nazis get kicked out or their audience leaves. I never mentioned any real world consequences (although they definitely exist too), but even if you purely talk about discussion quality and ability for people of different kinds to participate, you usually have to be kicking some nazis out eventually, because they tend to become as overbearing of a voice as the moderation lets them.