You can be compelled to act exactly as you would as if the NSL had not been issued, and indeed they seem to contain language to that effect. A reasonable response would be to adopt a policy of "no comment" and most do, but if you have previously responded to questions about NSLs in the negative, you would then be required to continue doing the same.
Q: Have you received an NSL?
A: No.
Q: Now have you received an NSL?
A: Still nope.
(Government issues NSL)
Q: What about now?
A: No comment.
^ This would be a clear canary indication that an NSL has been issued, which is in directly contradiction of the terms of the secrecy requirements laid out in the NSL. The only reasonable response (aside from supporting the ACLU and EFF to end this nonsense) is to adopt the "no comment" policy early, which most do. However Apple has now broken that strategy, if they haven't already.
Canaries only make sense if the company actually cares about letting their customers know about a possible NSL. They might not be legal as it's a pretty obvious wink wink nudge nudge that they received one. Apple and Amazon are big enough that they aren't going to mess around with utopian EFF rhetoric and will do whatever is required them by the law. None of the big boys care about "don't be evil" mantras anymore. They care about what keeps the stock price up and the federal government off their ass.
Do you have a source for this statement? As far as I'm aware, you can be compelled to remain silent but not compelled to give a false statement.