As is pointed out in the wikipedia article, Warrant Canaries have not been tested in a court of law.

It is logically possible that they have been tested, and struck down, in a secret court.

Or there might be a secret law making them secretly illegal

I feel for something like this to have a chance of holding up in court, the company would have to be less up front about the purpose of it. rsync tells the user that if they miss a day, or it stops getting updated, then something is wrong. I think without statements like that, and calling it canary.txt, they would stand a better chance of holding this up in court.

That's just my opinion though.