It's extremely illegal to collect on US Persons. The permission to do so comes from so far up, you'd never believe it, and it's on a case-by-case, person-by-person basis. It's so rare it's nearly unheard of. Most of the stuff they're talking about really is just passive collection. Basically, think of it as being like the cameras TV Stations have along major highways, or ones outside a gas station watching the pumps. If you do something wrong, they'll go back and try to catch it, but no one is sitting there watching what you're doing.
I know it's a little hard to believe, but protecting your rights is a huge concern, and one of the first things you get taught. Plus they all have intensive training periodically and lots of oversight programs in place to ensure nothing illegal happens. Actually, with all the movies that have come out to make folks so paranoid, it's an even larger concern than it used to be.