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I think for some it probably is the anonymity of the Internet. I hadn't taken that into account, but now that you mention that it makes sense. Out in the 'real' world there are potential repercussions for expressing extreme views in the work place, you don�t risk this unless you are secure that your views will be accepted and validated, that being in a recognized social grouping that has already �rules� about what is acceptable or not.
In social groups there is the same usually unwritten rules which allows you to become a member, excludes you if you do not fit in, or you decide the group is not for you. Given my age and profession I am not constantly meeting new people so I am probably not as regularly exposed to the continuum of opinions as I am on AB.
So AB allows you to have free reign without inhibition because there is no real public sanction or condemnation. Wonder if this is a healthy outlet or not.