Hmm... Why not, exactly?

If I'm investigating you over a bomb threat I'm not going to tell you beforehand so you erase any evidence.

People have already found and reposted the bomb threat posts, so that argument makes zero sense.

Why national security of course.

That's fair, actually; if that is the case then I very much hope to see a post in the near future explaining what happened as soon as it's no longer an active matter for the police.