To your point, if there is indeed a line, it need to be clearly articulated so that the rules of the game clear.
Try to draw the line wherever you want, but they're going to step over it, and never look back.
Especially today, I think we have to look at every power we might give to government and ask, "What happens if the worst people get access to this?" Because they're certainly going to try.
Based on what?
From my perspective, the easiest way to design such a system would be to create entries for every 'actor' in the system, feed in as much data as you can get your hands on, and then let the weights sort themselves out. So for example, if you're hanging out with ISIS members obviously your weights would be higher, but even if you're a server at Applebees you'd still be in the system somewhere.
Doing it the other way necessitates some kind of bright-line division, and any such boundary, once defined, becomes susceptible to exploitation. e.g. I won't hang out with the White Nationalist Militia because that puts me "into the system", but I can hang out with insert radical right-wing group where I can talk to 80% of the same people without being flagged. In practice, I imagine that the gradient of extremism is rather gradual and with blurred boundaries.
Perhaps you think all FISA rulings should be public and any sufficiently savvy malicious actors can just read them to know exactly how to avoid suspicion?
The FISC process is used when the purpose is foreign intelligence rather than domestic law enforcement, and it exists because prior to that there was no limit on the covered activity when it was done for that purpose.
> Precedent exists, but nobody outside the national security state actually knows what it is.
Well, some of it.
https://www.fisc.uscourts.gov/public-filings
https://law.justia.com/cases/federal/appellate-courts/fiscr/