I'm not sure what the reason was here, but on other forums I used to frequent they disabled the edit after a short period of time so that those who might want to reply don't waste their time on a 5 or 10 minute rebuttal only to have the OP delete the original thread.
One of the oldest forum spam techniques in the book, is to make a bunch of relevant posts, then go back later (like a week or two later) and edit the post to insert your money links.