It's important to remember that basically nothing has a zero probability and anything that's happened once has some likelihood of happening again---you can decrease the probability but you can't eliminate the possibility. At that point, it's a cost-benefit analysis problem (and a resource management problem). However, whether you're talking about air terrorism or health care or child education or anything else, it's often hard to convince a general population of this... and it often ends up seeming like you're trying to throw someone under the bus if you even try. sigh