Comes from an understanding of how service providers actually get gagged: the courts actively issue a gag rule based on orders they themselves issued. That's can't be true of basic science.
They could try passing a law outlawing security research, but: good luck with that.
http://en.wikipedia.org/wiki/Born_secret
The NSA isn't going to swoop in and claim that the results of the Truecrypt audit were born secret. But the answer to nabla9's question is yes, legal precedent exists in the United States for restraining the publication of original research when it is perceived to damage vital national security interests.