Unmod originally allowed people to post as anonymous guests. Some people abused this "privilege" and floodspammed the forums with it. DJP turned it off to keep Unmod a bit cleaner and to keep the trolls out.
Later on, Unmod went a bit more "adult-only" so to speak and it being visible to everyone who visited wasn't helping much. It got hidden for registered members only to keep the random people that just happened to pass by a bit safer (IE those only showing up to see what happens on the OCR forums)