You must be missing the point. The point is to collect his posts in such a way that keeps a record of his interactions. What good is a record if he himself can edit them?
if he edits them it would become evident if they were stored elsewhere on the forum (or as many have pointed out) on another forum site.. With the internal feature it is not possible at this time to secure posts from editing either at the user level or (if as claimed) the moderator level.
I didn't miss anything.
(also for the record, I would like to point out that BFL_Josh/Inaba has been much nicer since this was proposed. Either that or the Mod talked with him about his dramatic behaviors.
I also noticed that a frequently unused account called BFL has become active. It averages only a handful of posts (1 or 2) per month. It is very curious to see and leaves me in wonder if there will be new names and handles.