Listen to activity where one party to the discussion is reasonably suspected of terrorist activity.
Don’t “Listen to everyone, just in case someone does something wrong”, which is the current approach.
And then collect all data related to that party...all calls made within the last two years, etc. in order to catch not only the one party but the whole network. You can't do this if surveillance is curtailed.
Knowing the US can do this is keeping the terrorists at bay.