The financial markets depend upon stability and law and order. They cannot function without it. The fear that markets and industries could be crippled with a simple bomb (not due to the reaction, but due to the event) did massive damage to the markets, which resonated and caused significant economic collateral damage. This is obvious given that the reaction to 9/11 was actually quite sedate for some time, and massive enterprises like the NSA's current adventure came about much later.