The issue with an 'enter at your own risk' forum would be, where would the line of harassment be drawn? At that point, the forum must then regulate what goes on in that forum, because while it is a flaming forum, it could to get out of hand. For instance, racism shouldn't be linked to any community for any reason, regardless of where it is, because it'll just lower the respect of the community.
Besides, if someone gets angered in that forum, they'll likely leave the site before making other contributive posts elsewhere.