There are some threats that are so great that, no matter how improbable they are, there is no limit to the resources that should be expended or liberties curtailed to mitigate the risk they present. 

In such cases, the government must immediately do something, anything, no matter how blindingly inane, as long as it is in some way related to the threat.
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The problem is that there is always little objective knowledge on said threat.  As such, some people believe these threats have never ever occurred in the history of their country.  Others, conversely, insist that every single threat (no matter how meagre) is great enough to require this sort of approach.