This implies, without evidence, that all of the work that NSA does is directly related to keeping us safe. I wouldn't be so quick to lump it all together like that.
Then, of course, there's also the objection that "keeping us safe" is not an absolute. There are many ways to keep people safe, but they're so extremely onerous that we don't practice them. Totalitarian societies with pervasive open surveillance (think 1984) are very safe, for example, but at what cost? So clearly there's a balance, and one can't just dismiss any concerns about the cost of that safety by saying that it's necessary - it has to be demonstrated that it is (i.e. that the gains from that increased safety justify the losses from intrusiveness).
Then, of course, there's also the objection that "keeping us safe" is not an absolute. There are many ways to keep people safe, but they're so extremely onerous that we don't practice them. Totalitarian societies with pervasive open surveillance (think 1984) are very safe, for example, but at what cost? So clearly there's a balance, and one can't just dismiss any concerns about the cost of that safety by saying that it's necessary - it has to be demonstrated that it is (i.e. that the gains from that increased safety justify the losses from intrusiveness).