In computer security, closing down the entire system when faced with an incoming attack is stupid. If you can trigger the shutdown easily, then you're effectively denying service to the system - a DoS attack. There's obviously a line between destroying the system and making it inaccessible, but the end result is mostly the same.
Dummy explosions shut down London.
The terrorists have already won if they can force us into submission through a few well placed empty bags/telephone calls, and we're so gullible that we believe that the only way out is to keep a stuff upper lip.