This is very misleading, mainly that is ONLY considering the loss of life from terrorism. Certainly the 9/11 attacks were NOT a major per capita killer. Howerver, what about the other effects:
* Economic value of destroyed property: these attacks brought major damage/destruction to three of the most expensive structures in the entire country. The cost of the attacks alone cost billions and billions of dollars. This was not a bus blowing up where the loss of life is comparatively high in a small state in Israel but where the cost of destroyed infastructe is minimal.
* Symbolic value of destroyed the most important landmarks and the effect on the country's psyche. Equivalently, by this argument you could say presidential assassination risks are minimal because they kill 1:300,000,000. But the symbolic and resulting implications of the TARGET are massive.
* Economic Results of Attacks: Such large scale attacks crashed the stock market, nearly bankrupt the airline indursty (or did), and contributed to widespread fear and instabilty. THe cost of future attacks could have brought down the entire system.
So no, they did not overreact (aside from the stupid foray into Iraq, but that was just Dubya being Dubya)