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But are we sure the Court was able to view everything it needed to know these five do not pose a threat?
The whole problem revolves around the (artificial?) distinction that has been made between terrorist activity and criminal activity. I'm sure the FBI and other civilian law enforcement agencies would find it very useful to keep all their tactics and intelligence sources secret, to convict mobsters without letting them know the evidence against them.
Wouldn't law enforcement be better off if citizens had no idea how the FBI gathered evidence? Even serial killers would have a harder time covering their tracks. Every time a law enforcement official testifies in court, or the State details its case against some accused person, this is of value to potential criminals. Yet society insists upon the state making these revelations.
I'm sure right now the FBI knows about dozens or hundreds of horrible criminals who are free in the US, but cannot be imprisoned because they don't have the evidence to convict them, or would prefer not to out informants and sources near to these people.
So why does society allow these people to be free? Many of them will kill innocent people or commit other crimes.
It is because we know the people running the FBI and justice system, however good intentioned, cannot be trusted to remain that way if given this sort of carte-blanche to "get the bad guys" - that it is better, overall, to let a few bad guys roam free, than risk the abuses we know would happen if law enforcement had these kinds of arbitrary powers.
Then why should prevention of terrorism work any differently? Are DHS employees and soldiers some kind of perfect humans who never make mistakes?
The government has to be willing to show its evidence or let them go free. It is a hard choice, and there is some risk either way, but for a free society, they can't have it both ways.