For what it's worth, everybody, not everyone agrees with all of that. The forums are a valuable tool, and while they're not the main focus of the site, I personally disagree with the attitude many (moderators/admin and others alike) harbor, that the forums deserve less attention than the rest of the site simply because they're not the main focus.
Having said that, what Lou said about the reason for disabling editing is true and a very good reason. There have been many cases of people saying something incredibly stupid, and being taken to task on it; then they come back and decide to delete what they posted, as if that will undo their idiocy. I am a veteran of MANY message boards, moderator and admin on quite a few of them, and not many of them allow you to edit a message after a) an hour or two or b) after a reply has been posted to your message. That's how I think it should be by default.
I also dispute the claim that there is no excuse for double posting. Often, someone will post something while you are posting, and that new post warrants a reply. I suppose that could be done with an edit to the original message, yes - but keep in mind that most of us that use notification of new replies use the "instant" form of notification. As soon as the message is posted, we're sent an email saying so. Notifications aren't sent on replies, so if you make an edit, I won't see it if I've already read the original post. (On that topic, the text of a new post is published in the "instant notification" message. A number of times I HAVE seen people who have said something and when I go to read the message, the sentiments are quite different! Everybody please remember what you say here is broadcast around the world.. if you don't mean to say something, don't say it at all - instead of saying it and then editing it out of your post.)
And as for another scenario that was mentioned, I'm all for "double posting" when you have information to add 24+ hours later. I see no reason to be editing the original post in that case. For one thing, the original post should remain as it was originally posted, unless there are spelling, grammatical, or typographical errors that need be fixed. (As Lou said, use the preview function instead - that will help a lot.) Another point to ponder is that replying to the post will "bump" it to the top of the "newest posts" stack - editing it does not. If you have a thread that you want to supply new information to, wouldn't you want others to be able to see it?
In summary - the "post editing" policies of this site are quite appropriately defined and deployed as far as I'm concerned. Those who have need to edit a post outside the editable time window need to evaluate whether or not they really want to edit, or could simply reply instead.