Re: "an indispensable tool when it comes to fighting terrorism"
SLA (1973). Bath school massacre (1927). Harper's Ferry raid (1859) – whatever your opinion of Brown's motives and whether the wrongs he wanted to right justified it, that was basically a terrorist attack. Maybe Shay's Rebellion (1786).
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The American Revolution arguably wasn't a terrorist operation prior to its success, because it didn't have an explicit aim of advancing a political goal through threats of violence, but it wasn't all that far off. You could make a similar argument for some of the above, but the line between "terrorism" and "just regular ol' wide-spread violence and murder" gets blurry.
Even with a strict definition of terrorism it's hard to see how e.g. the SLA, Oklahoma City, Unabomber, and DC sniper attacks don't fit.
That said, Pascal's point that we lack public evidence to support the FBI's claim that they're stopping All The Terrorism seems pretty valid. You want §702? Show us something real. And even then, it needs to be scaled back dramatically.