Why are you so convinced that terrorism is not a threat to society? Lots of Romans probably thought the Huns were a laughable foe as well.
Terrorism is not Effective: http://www.gwern.net/Terrorism%20is%20not%20Effective
Terrorism is not About Terror: http://www.gwern.net/Terrorism%20is%20not%20about%20Terror
Just look at the statistics. The total number of people killed by terrorists is negligible. And 0 terrorist groups have ever won and achieved their stated goals.
I remember the Boston bombing. That same week, there was another explosion at a chemical plant that killed many more people, and blew up a nursing home. Guess which event got more press, by a large amount? Which event are people more likely to remember? Can you even name what city the chemical plant exploded in or remember any details about it?
Why is this? It's because people react to violence much differently than they react to accidents. Because thousands of years ago, violence was the one cause of death you could do something about. If you got an infection, or if a natural disaster struck, or a famine broke out, etc, those are all out of your control. But if someone was a threat to your family or tribe, you could fight them, or flee from them.
We have many emotions and social instincts whose entire purpose is to deal with other humans that threaten us. Like paranoia, anger, and hate. No one hates the chemical plant, and wants to go kill it. People hate the Tsarnaev brothers.
Don't speak too soon. We have several political movements dedicated to this.
Hmm. Wasn't it Bin Laden's goal to restore the Islamic Caliphate?
It's all about human agency and intentions. Terrorist attacks are carried out by HUMANS with the clear INTENTION of inflicting harm on the populace. Accidents on the other hand lack these essential factors. When I walk by a chemical plant, it never crosses my mind that the plant would attack me or be capable of doing such a thing and therefore I am not worried to be around but on the other hand if I happen to be walking in a bad neighborhood late at night, I would be very wary of people around me because I know very well that some of them might inflict harm on me for any reason.
So, I think that the comparison you drew between these two categories is not that solid and that people have every right to be "terrified" of terrorism and take all the precautionary measures to stop/prevent it.
Blow up enough people often enough and fear and panic will become pervasive. People stop opening their shops, investments stop coming.
Terror is a pretty effective way to overthrow a gov't.
Another is South Vietnam.
Although there was a war going on the NLF did a great job of terrorizing the local population and installing their own shadow government. The terror attacks fractured the populations allegiance to the South Vietnamese gov't (what good is a gov't that can't protect you?).
Terrorism can be an effective tool in guerrilla warfare.