The issue is that once someone has posted and everyone's replied to them...If they then go and change the original post, suddenly all the following posts are pointless for someone reading the thread without having realised that there's been edits.
Someone could say one reactionary, cause a stir, and then change it to something innocent later in the day (or week) and everyone else could end up looking like they are trying to start a fight over nothing. (Not that we EVER start fights...*sharpens said pitch forks* ...ahem...)
Or, with fanfics, once someone's commented on a certain scene, if later you were to decide to cut the scene or move it, the conected comment would be null and void and confusing.
Even little things like spelling and grammer (Sure, I'd love to go back and fix up my first few weeks here at MC when I had teRible spiLlgn...), if something is changed at a later date...it could make things very illogical for anyone who maybe replied with, say, the correct spelling of Bovegard...I mean Beauregard...