It is
not settled law in the U.S. whether being forced to handover a password is "contempt of court" or actually self-incrimination protected by the Fifth Amendment.
Wiretaps ordered into the U.S. phone and internet networks via CALEA and FCC rulings, which basically meant that all network and phone switches internationally would have the back door. (See what happened to Greek politicians thanks to that). But the phone portion of those rules came from a very deliberate act of Congress. (Don't get me started on CALEA and the Internet...)
The All Writs act is vague as hell.
And there are definitely laws that require legal process; your telephone records, for one. That's why the Congress passed a law retroactively immunizing AT&T, Verizon, et. al. for helping the NSA collect communications data on American citizens in plain violation of communication privacy laws.
In this case, NOTHING is straightforward. Apple could win on First Amendment grounds; the assistance asked for could be determined to be too burdensome. The Fourth Amendment may not even be an issue.
I'm not even sure this will go to the Supreme Court. If the feds lose, they may well choose NOT to appeal on purpose so they can continue browbeating companies with All Writs act in other jurisdictions and under seal.