MathFox I completely disagree that it is acceptable for a terrorist act or murder to happen. It is OK by me if a few people are treated unfairly for the safety of the rest of us.
Scarrymike, if you want 100% prevention of murder, that would mean preventive solitary confinement for 100% of the population. I don't think that is achievable. So society accepts a few violent deaths per million people per year and has some rules for prevention (a ban on firearms) and punishment (justice).
In engineering we commonly do FMEA (Faiure Mode and Effects Analysis). You multiply the chance of a failure happening by the likely damage caused, and modify the design if the number is high enough. The exception is when a failure however small would have disastrous consequences (major loss of life). In that case you change the design - in this case taking what might be seen as excessive action.
I am familiar with FMEA; I agree with some of the measures taken (reinforced cockpit doors, metal detectors) and think it's time to focus again on "pilot error" as leading accident cause. On the other hand, investments in road and car safety will prevent more damage per "buck" than investments in air safety. Yes, the road is one of the biggest killers (behind the bathroom).