Emphasis is Thiessen's. Here's the poll, and the full wording of the question that was asked:
2. Are you personally worried about traveling by commercial airplane because of the risk of terrorism, or do you think the risk is not that great? (IF WORRIED) Would you say you are very worried or only somewhat?
If two-thirds of Americans aren't worried about their flight being part of a terrorist incident, it's because they're being entirely rational. Nate Silver actually did the odds on all of this about a year ago:
The odds of being on given departure which is the subject of a terrorist incident have been 1 in 10,408,947 over the past decade. By contrast, the odds of being struck by lightning in a given year are about 1 in 500,000. This means that you could board 20 flights per year and still be less likely to be the subject of an attempted terrorist attack than to be struck by lightning.
Now Thiessen perhaps adhere's to Dick Cheney's "one percent doctrine" approach to security matters, which suggests that if there's a one percent chance of a bad thing happening, the United States should treat it as a total certainty. Such thinking led us into a fruitless war against Iraq's WMD program, of course, but nevermind. If one accepts that doctrine, then a person's odds of being on a flight involving a terrorist incident are substantially less than one percent. The disparity between the threat and the resources being marshaled to meet that threat, then, is nearly infinite.
I'm not suggesting that a terrorist incident involving a plane can't or won't happen. But if the last decade has shown us anything, it's that A) it's hard to hijack or attack a plane in the post 9/11 era, because attackers who get past security still have to deal with the passengers and crew of a plane, and boy do those people want to live; and B) there's probably not a real large group of people interested in attacking us anyway. Thiessen gives terrorists too much credit, and the American people too little respect.