The US has organizations which look inward for terrorists - the FBI, DEA, ATF, TSA, local law enforcement, etc. We have our own country fairly well covered, with well-establish legal precedent and rules of engagement. The NSA is an intelligence gathering organization for the parts of the world we don't have covered, and its goal is to collect global intelligence. It doesn't treat anyone adversarially, as it has no "troops" with which to engage foreigners.
It's massively near-sighted to think there are no groups in the world plotting the harm of US citizens. You might not be part of those groups, but how do we know if we don't look at you?
> it has no "troops" with which to engage foreigners.
Well, if you ignore that thing where the NSA is an part of the Department of Defense, and that its director is also the commander of a sub-unified command of the US military, sure.
It's massively near-sighted to think there are no groups in the world plotting the harm of US citizens. You might not be part of those groups, but how do we know if we don't look at you?