I want to point out that "fair use" or, even more generally, First Amendment rights, are not involved here.

The statement that the NSA issued, that no one is allowed to use the Seal without written consent, is strictly false.

Section 15.a states explicitly that you may not use the Seal "in a manner reasonably calculated to convey the impression that such use is approved, endorsed, or authorized by the National Security Agency."

This t-shirt does not give you the impression that such use is approved, endorsed, or authorized by the National Security Agency. Fair use, parody, satire, doesn't matter.

I linked the CNET article because the FBI did the same thing a few years back:

"While we appreciate your desire to revise the statute to reflect your expansive vision of it, the fact is that we must work with the actual language of the statute, not the aspirational version of Section 701 that you forwarded to us," Mike Godwin, general counsel for Wikimedia Foundation, the nonprofit company that runs Wikipedia, wrote the FBI in response.

They omitted a large part of the law in their statement. You can't say "Endorsed by the NSA" on your product. You can say "The NSA Sucks." The use of the logo is allowed in the latter.