Earlier quoted context omitted.
How do you imagine that working? There's a law that allows them to ban disclosure under penalty of law. Is there some other law that allows them to compel people to lie?
There's no law or constitutional amendment _preventing_ the government from compelling people to lie, is there? In the contents of an already-secret national security letter sent to rsync.net, what prevents the government from saying "You may not tell anyone about this letter; this means you must continue updating the canary as if you hadn't received this letter"?
Congress, in the USA PATRIOT Improvement and Reauthorization Act, has said that the feds can ask for records, and can additionally forbid people to say that they have been asked. It doesn't say they can compel people to lie.
If the government orders them to do it, they can take the government to court, or they can just say no. The government then has to arrest them and try to convict them. Either way, a court would have to decide for sure. In this case, probably the Supreme Court, in that compelled speech has often been ruled as against the first amendment.