Seriously, I believe the authorities are well aware that assembling a bomb would have little overall effect to a terrorist. They wouldn't be able to hijack a plane with one, or with a razor blade even, since that could only work on one horrid day. No one would believe them if they were to say to remain calm and everything will be okay. A bomb on a plane can kill a planeload full of people, but that is small potatoes to a bomb placed in a baseball stadium, or a busy street corner at lunch hour.
Perhaps, Islamacists wish to blow up a plane precisely to kill Americans on the plane and not for any other reason. Certainly, this was the intention of the shoe bomber. Consider the intifada tactics of the Palastinians... Terrorists wish to well...create terror. If a plane is blown out of the sky by terrorists, it will certainly frighten people (to say the least) and cause the general public to consider that perhaps we can not stop terrorists. This would certainly affect our morale and our economy..people will stop flying.