I think there's also a very reasonable fear of making a false accusation - what if you're wrong, and it ruins the life of someone you're likely quite close to?

I think there's also a very reasonable fear of making a false accusation - what if you're wrong, and it ruins the life of someone you're likely quite close to? Especially if an arrest is done very publicly, as it might be in the US, with SWAT kicking down their door, and a story in the news. Sure, the person might be released without charges, but that never gets the same attention as the initial arrest. When the persons name comes up in conversation there'll always be "isn't that the guy they arrested for maybe planning terrorism?". Who wants that guy for a colleague or neighbor?