Some Americans think working for the NSA is ethical. I'm an American engineer, and I would not work for them, given their recent history.
I would be surprised if you found an American working for the NSA who didn't do so willingly-- people in that position can easily find other jobs, so they do.
People think working for Goldman Sachs is ethical. People participating in the Summer Olympics think Russia is ethical. People buying Nikes and Fuji water think they are ethical.
You don't think you're biasing your analysis by concluding that every NSA related person believes in what they're doing? People that don't believe in what the NSA is doing wouldn't work there. Its going to be very hard to find someone who doesn't believe, since adults that change their long-held opinions are pretty rare.
To name one that did change his opinion of his "mission," Edward Snowden.
Even the Guardian is not clear on this. Does the Guardian support the existence of the NSA, or not? You can't have your cake and eat it too.