Since 9/11 there have been a couple of attempts to blow up planes. Both the shoe bomber and the underpants bombers were on flights that originated from foreign flights. Despite that fact the TSA have stepped up airport security to a level of paranoid hysteria.
There are an average of over 25,000 domestic flights per day in the U.S. That's about 9 million flights a year and over 70 million since 9/11 yet there hasn't been one attempted on flight bombing on a domestic flight.
If a terrorist in the U.S. wants to randomly kill Americans the last way they would do this is on a flight. The security rules in place even before the latest security methods were more than enough of a deterrent.
Sports events, concerts, shopping malls, office buildings are all tempting targets where there is a minimum of security. Terrorists want to cause hysteria and fear. There are so many other avenues where they could do that without detection so any potential terrorist already inside the U.S. would have bringing down a plane at the very bottom of the list of juicy targets.
Are we that stupid? Obviously we are.