I understand the concept, but discloses something you can't disclose. They can compel you to lie/not comment if asked, "Hey, Apple, did you get any of those National Security Letters".
Is there a clear cut loophole or is this something yet to be challenged?
While there's plenty of precedent for gag orders, there's not much case law for compelled false speech.
Until they have been served a warrant, they are not under a non-disclosure warrant. That's how the canary is legal.
> hey can compel you to lie/not comment if asked
No, no they cannot. They can prevent you from commenting, they can NOT compel you to lie.
Lying and not commenting are very, VERY different.
The Federal Government disgrees with you. Just one example:
https://www.google.com/webhp?#q=least%20untruthful%20manner%...
It has nothing to do with being compelled to lie by a court order. It has nothing to do with the Judicial system at all. It has nothing to do with lying vs. not commenting.
A legal system can't let law enforcement officials decide after the fact what's permitted and what isn't (that's the theory anyway; your actual experience may vary widely).
So if a law forbids you to tell something, but doesn't explicitly forbid you to not tell another thing, the non-telling of which could potentially reveal the thing that's supposed to stay secret, then you can claim that you technically obeyed the letter of the law, if not its spirit.
So no, it is not perfect. But it is better than nothing.