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a well-thought-out weakness can easily be worth [a lot]Indeed, but my point is, in addition to be worth a lot, it's difficult to implement, and it's very fragile. Every time they use it, they gamble its secrecy, hence its effectiveness. So they won't use it for petty reasons, only for genuine national security matters.
The problem isn't that NSA works on ensuring national security: it's that the scope of what they consider national security, and thus justifies extra-legal measures in their eyes, grows unreasonably, becoming a threat to the robustness of democracy.
A spy agency's ideal environment is totalitarian, not democratic. For a starter, they're scared of free speech and accountability. There must be counter-powers, cancelling their natural tendency to push towards totalitarianism (this is in no way specific to the USA).
> If they decide they don't like you there are all sorts of other ways [than courts] they can screw with you.
Yes, but those ways don't scale. You can screw with a couple of people you dislike, but not with hundreds or thousands of people simultaneously.