It's not every day that people have their legs blown off while enjoying a public social event.
There's an expectation of danger while driving a car. There is no expectation of danger while spectating at the Boston Marathon.
It's not complicated.
You are in the presence of many other human beings. You are always in a present state of danger around everyone, ever. It is naive to think you can predict the actions of others.
I'm not saying be paranoid, I'm saying treat people like people and not faces in a crowd. You don't know their intentions with rare exception to those you actually know.
There also is no expectation of getting struck by lightning while spectating at the Boston Marathon, yet that is much more likely to happen than getting hit by a terrorist's bomb.
It's an obvious appeal to emotion that doesn't refute the point that terrorist attacks are so infrequent that the average person ought to worry more about, say, slipping and falling in their bath tubs than getting killed by a terrorist.
So I suppose this would be a red herring on your part, if we're accusing each other of logical fallacies.
Unless you're trying to say that the odds of being struct by lightning while spectating the Boston Marathon, on a clear sunny day, is high.
Not sure where you're getting your data from.