The coders added the "edit" function because users were complaining that they spotted typos or mistakes right after they hit the "reply" button. I.e. users didn't bother to hit the "preview" button first. So the coders added the "edit" function as a way for users to immediately edit a bad post. The main reason for no delayed edits is that it would cause exactly the kind of confusion you mention. If you post a new topic, then 30 or 40 users add their posts in response, then you change your original post, it would tend to make the other 30 or 40 guys look like idiots who had responded to something you didn't say. It's just as confusing to delete the original post, so the only non-confusing method is to let it stand, unedited.