That's what I thought, too. Using "The NSA" in the way the shirt does is making a political statement, which is protected by the USA's First Amendment. It's one of the few rights that the USA Supreme Court has defended zealously.

Supposing that "The NSA" is some kind of "intellectual property", using the phrase as that tee shirt does is almost certainly fair use. This is a lot less certain, as you'd have to go to court, spend lots of money, and admit to infringing to plead fair use, so it's probably not worth the effort.