User blog comment:DaNASCAT/Technical Update: August 16, 2011/@comment-3971662-20110818023533

Why not have an optional restriction that anon/new users can only add categories from the auto-suggest? If they try to create a new category, it can link them to a help page that explains how categories work. Once they're three days old (and hopefully know how to properly use categories), they can add the new category. Activating the restriction can be up to an admin and it could be accompanied by an explanation of why the option exist. It would likely only be used by experienced admins who know what the problem is and all categories would still appear as blue.