Take a gander at some drone videos out of Syria and Iraq. This is a very real threat. There are countermeasures that I no doubt are deployed at major American events.
Take a gander at some suicide car or truck bomb videos out of Syria and Iraq. That is a very real threat.
Take a gander at some videos of terrorists driving cars or trucks into crowds. That is a very real threat.
Take a gander at the images of airplanes hitting New York. That is a very real threat.
The problem isn't the machines. It is the people who decide to take a very useful machine and apply it to fulfill their evil intentions.