Under US law my understanding is that your password is something you know and thus can't be compelled. (Although border control can say that showing them your phone is a condition of entry, but that's only meaningful for a non-citizen.) He's a citizen, they denied him access to the attorney that would have told him he didn't have to comply. But note that your fingerprint is something you have, not something you know--they can press your finger against the phone to try to unlock it. That's part of the reason your phone wants the PIN after reboot.
The distinction between physical and legal presence is something that most people don't think about but it's effectively impossible for society to operate any other way. With land borders you technically could put the border station exactly on the boundary, but that would be quite problematic and is never done. There is also the problem that most countries have exit controls, also--after you pass the exit control you are no longer legally in the country even though you physically still are.
Then when you throw in reality it gets even more complex. I have been on a plane that skipped landing in Taipei and proceeded to Hong Kong (I won't call it diverted as the flight was Tokyo - Taipei - Hong Kong, we were just going to get off in Taipei.) Passports never looked at, we were shuffled around through the secure area of the airport and put on another plane that was flying the reverse route. And I have made a transit of Botswana by train without ever encountering any officials from Botswana. The only officials we saw were from Zimbabwe (where we boarded the train) and South Africa (where it was going.)
And we got caught in no-man's-land between Rwanda and Congo. Cleared Rwanda exit control, proceeded across the border (land crossing) to Congo, they said sorry, border's closed until after the election. We had a still-valid but useless visa for Congo, our visa for Rwanda was single entry and had been used. The Rwandan officials saw the problem and cancelled our exit from Rwanda.
And I have done US pre-clearance from Canada, legally I was in the US, not Canada, before boarding the flight. This does more than simply move the officials, the flight was treated as a domestic US flight and could land at airports without international facilities. And, back in the less formal days we ended up going through Singapore without our passports ever being seen other than in our hands. (On arrival, we lost an object on the airplane and spent a while hunting for it, by the time we got to the officials they were done with everyone else, they were talking to each other and just waved us through. Then, on departure the idiot taxi driver took us to the cable car station instead of the train station. The train started to roll while we were still in line--several of us with first-world passports got waved through. The train was moving slowly enough we managed to catch up to a door before we ran out of platform.)