Their claim is that it's in order to listen to the conversations where a foreign person of interest is the other party. :)
Don't get me wrong, I don't like the surveillance either, but domestic terrorism isn't their job. Sure, if they notice something, they will pass it along, but it's not what they are looking for (I believe). If there's been a proverbial ball dropped, it's probably more accurate to blame the FBI than the NSA.
Domestic terrorism isn't the NSA's purview. Their job is signals intelligence against the rest of the world.