Because the Policy does state the thread will be deleted.
Quote:
Excessive BTT/Bumping post will also cause thread to be deleted. If user continues to BTT/Bumping threads excessively, user will be banned.
As mentioned above, numerous places to find the Policy - those are the warnings.
That is one of the reasons why we posted it in the
Site News and Announcement section along with t
he Click for Classified Ad Changes Effective 12 Sep 2011 link at the top of EVERY forum page.