It is not irrelevant at all. It is to prevent people from editing insults, forum rules breakages, etc. out of their posts before the moderators see them.

If a flame war breaks out, and one of the participants goes back and edits all his posts, he can claim to be innocent and there is no way for the mods to prove that he actually wrote the offending messages his opponents quote. Therefor known offenders have their edit button disabled. A side effect is that it forces them to think before they post, since they cannot go back and edit it. A lot of insulting messages result more from carelessness than from bad intent.