I don't think the warning is clear enough, it should read something like this:
High scam risk! This is a self-moderated topic. The original poster can delete replies, including ones which point out fraud or dishonesty by them. If you do not want to be moderated by the person who started this topic, create a new topic.
Because on past experience that's what this is going to be used for, just like the local thread rules were.
Edit: The current warning also doesn't mention that deleted posts disappear completely, so there's no way for normal users to tell if the feature's been used to delete posts.
Makomk is 100% right.
Seriously Theymos, sometimes I wonder if you like having the forum full of scammers. Anyone with a basic knowledge of financial history knew Pirate was a scammer. How much more money would he have succeeded in stealing off participants here if he could delete posts, and use his cadre of believers to keep his threads at the top of the page?