I had a look at that recently. At some point, which I think I missed, the sign up procedure was changed from admin approval to automatic acceptance.
Previously, people known to admin were given wiki editor status when they were approved. Now there's no approval, we don't want just anyone with editing rights, so recent members become "Barrell scrapers".
Unfortunately, not too many people contribute to the wiki, so not having editing rights shouldn't be a problem.
Anyone wanting to edit or contribute to the wiki, who isn't a "Wiki editor" only has to ask and we can alter their status.