a couple of scenarios as to why this is a rule:

1- If anyone has quoted your post before replying to you - although YOU delete it, THEIR post would still have the information. So if they quoted what you wanted deleted its not truly deleted. Because its their again.

2- you find a thread you want to reply passionately to. You spend about an hour and a half researching and replying to the thread because you feel so strongly. The next day you come back to find it has dissapeared, and you have no idea where or why. You feel upset and confused, and end up having to pm a moderator to find out that the person deleted it.

3- its just not appropriate for anyone but a moderator or admin to be editing or deleting another members posts or threads. Can you imagine the amount of fighting that it could cause like number two, if you put alot of work into a thread, and someone deleted it..well, your probablynot going to be happy

4- sadly, there are sometimes a few rotten apples on the internet. There are those who would use it to hurt or antagonize others for whatever reason.It could be used to insult someone, call them a name, threaten them - until the user replies to them that they are reporting the thread- then they delete the original post, and there is no proof of anything.

Just a few examples to help explain!