Suppose I do something the Gubmint doesn't like. Perhaps organize a protest or start some sort of organization that threatens their power. In doing this, I probly won't be using stolen prepaid phones or hiding my message in every 13th word of a meandering email. Thus, unless I plan to hide my activities from the beginning (something I might not have contemplated before the present revelations), they'll have open and readable communications from me. Once they've identified me as a target, they can go back & dig up all my emails & phone calls & listen in. They will also know whom I've been talking or writing to, & can widen their net to include them.
Then, if what I've been doing is legal but they still want to get me, they can sift through those same communications for evidence of any nasty habits I might have. Like maybe I visited some questionable porn sites or an Aryan Brotherhood site or "eco-terrorist" sites or something. They could use any such information either to blackmail me or to tar me as a pervert of some kind. Maybe I contributed $$ to NORML & wrote an essay somewhere about medical marijuana. Some court might decide that's probable cause to break down my door in search of illegal substances. If they don't find any, they can plant some. Maybe I also own guns, so they can get me on committing a drug felony while in possession of a firearm.
This kind of surveillance is much more effective against babes-in-the-woods than against trained, organized evildoers. It can be highly useful in suppressing dissent even if it doesn't have any use in preventing terorism.
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