It's really icky to punish someone for doing something that wasn't a crime at the time and place it was done. If you create a law that prohibits visiting a park at night, you shouldn't retroactively punish half the population for having visited a park at night. This stuff is UN human rights stuff, and often the norm in sensible justice systems.
Drawing on this, it's not currently illegal to publish a warrant canary. If it's not illegal, you can't be punished for it now. If you can't be punished for it now, in a sensible justice system, you can't be punished for it down the road either, as long as you stop doing it before it becomes illegal.
What they could argue is that by ceasing to publish the canary, you're committing the crime of communicating something you're not allowed to communicate. That, however, skirts dangerously close to forcing someone to lie.
This was my point, but the issue seems to me not about them forcing you to lie but rather about you setting up a system that you know will either force you to lie, or to break a court order by communicating something you were ordered not to. The only point of a warrant canary is to try to bypass the intention of a potential future court order.
Which is not illegal, and therefore cannot be punished. I wouldn't be surprised if they make warrant canaries illegal (they are already in Australia?) for this reason, and it sorta kinda makes sense, but it also sucks.
Edit: though I guess you could argue that "preparing to break a court order" is some sort of crime, but
1. It's in this case equivalent to making warrant canaries illegal, and
2. Laws that are punished only occasionally are the scariest thing.
Which opens a whole other can of worms. Telling a lie is not illegal. However the impact of that lie might result in loss to a third party - and that would need to be reconciled.
That said, meta-censorship is pretty icky regardless.
The data for the following users (as of December 31 2015) has not been subpoenaed:
/u/mike_pants
/u/the_starship
/u/SlimJones123
/u/n3rdopolis
/u/mrbabyman
/u/GreedE
...
Individual users could determine if they've been the subject of a subpoena and I suspect that kind of cleverness wouldn't impress a judge.