Security consultant Bruce Schneier writes regarding the trade offs one has to make in security when dealing with terrorists:
Security systems fail in two different ways. The first is the obvious one: they fail to detect, stop, catch, or whatever, the bad guys. The second is more common, and often more important: they wrongly detect, stop, catch, or whatever, an innocent person. This story is from the New Zealand Herald:
A New Zealand resident who sent $5000 to his ill uncle in India had the money frozen for nearly a month because his name matched that of several men on a terrorist watch list.
Because there are far more innocent people than guilty ones, this second type of error is far more common than the first type. Security is always a trade-off, and when you’re trading off positives and negatives, you have to look at these sorts of things.
It is for reasons such as this that one should be very wary of weakening rules such as the presumption of innocence, the right to silence and the prohibition of double jeopardy in order to make it easier to jail those suspected of crime or terrorism. These rules were put in place for very sound reasons. Get any changes to them wrong and the impact is likely to be felt mostly by the innocent. A safer strategy is to bolster the investigative powers and the resources available to the police and security agencies dealing with these problems.