This is a bad idea! I've had situations where people start a thread, and other people respond to it, and the thread starter changes the original post, making it appear to be a different topic altogether.
Example:
Thread starter starts a thread called "weather" and asks if the weather is good today. Everyone replies "yes, it's great. I love it".
Thread starter changes the thread title to "some weird sex act" and the first post to inquire about some weird sex act he feels the need to share with everyone else. It can turn ugly and make quite a few people angry!
The deletion and post editing defaults are the way they are for a reason