The point is that even if a few people die in various malls across America, people will be afraid to go there, stop shopping and it would hurt the economy.
You are right. A lot of the country quit looking at their mail for a long time.
Also, for low-tech terrorism, consider the two snipers who paralized a good portion of the east coast for weeks. Point being, it doesn't seem to take a lot to spook the public. A spooked public usually isn't a "shopping" public.