I understand the merits of everyone being able to edit their own posts, especially for sale ads, like when you want to change the price, etc..., but I can also see cases like dr1 has seen in the past where someone goes back and changes or deletes a post and nothing makes sense after.
Most of the moderation takes place as as new posts are made so edited posts will very likely not be seen by one of the mods. For instance, when we see a spammer it's when the post is initially made that we delete the post/ thread and ban the spammer. If they could edit later, they can go back and add their handbag and tennis shoe crap, and a mod would most likely never know. I'm sure there may be other reasons. It's been that way as long as I've been here (2007) and I never saw it as a problem.