What is that a threat to do?
How about, "Be careful."?
"Prior restraint" is a well-tested concept and it doesn't do very well in court. And all the government has to harass you are the courts.
Though I of course strongly disagree with your premise anyway, particularly when the governmental agency in question is the DHS.
There's a difference between "telling/informing" and mandating. The latter is bad; the former is...well, part of a government's job is to disseminate information.
The question then, is it in the public interest not to cover this story. I'd say it would probably not matter so much from a security standpoint, but more so for the amount of mayhem that could ensue at airport security if this was public knowledge.