Comment by neolog

The NSA's mandate concerns sigint, so that's what they do. Other entities (eg the PCLOB and federal courts) can constrain it, and the NSA constrains itself proactively to avoid embarrassments. The current result is a tradeoff between surveillance/security and privacy: NSA policy is that they can listen to foreign suspects' conversations, including if Americans are on the call. Metadata is collected on calls two "hops" from a suspect. Whether it's the right tradeoff is an interesting debate.