Just because it's not enabled in certain other sections doesn't mean that the Service Announcements should follow suit. Personally, if I were a thread owner in the Service Announcement section I would want to self moderate to prevent all the spam comments that these type of threads are subject too.
You are right. It's help to remove spam. If you or any well known managers open thread with self moderate then welcome. I have no complain about that. But once you visit that section I think you will not find 5% thread self moderated by any well known managers. Most of self moderate thread created by ICO team with their own account. That's the problem.