This isn't an argument in favor of the espionage state, upfront clarification. People typically aren't afraid of things they haven't yet personally been given a reason to be afraid of.
Americans are told the spying makes them safer. Americans fear terrorism, and they're told the focus of the spying is terrorism. Very, very, very few Americans have seen negative repercussions from the spying (so far as they personally notice or 'feel' going about their daily lives). So far, to that typical American, it's almost all theoretically helpful and not harmful.
I agree that a lack of understanding - properly comprehending the full historical scope and risks of such power - is at the root of why it's tolerated. I think inside of most Americans, you'll find a belief that their government can't be fundamentally evil (thus they don't fear the really bad potential outcome scenarios of such spying), even if it occasionally does something bad.
