At least agree that there is a conflict here; otherwise you're just being dense.
AFAIK the govt. cannot compel (legally) anybody to reveal the combination to a ticking time-bomb; or the location of a murder weapon etc. How the hell is asking for a passphrase any different? Do you really not understand the nuance here?
The other point is that AFAIK the govt. has never asserted the right to compel a handwritten note to be deciphered. The govt. doesn't assert the right to systematically read people's mail (even postcards); but they do so wrt electronic comms. I think we can safely state that almost all comm. is going to be electronic in the future. That's why this is important. This is yet another example of the govt. trying to trim back rights that the hoi polloi have enjoyed for a couple of centuries.
As to why this is important: you might have read the quote that six lines by any man's hand are enough to hang him.
Sorry, I meandered a little there.