Earlier quoted context omitted.
The same is true at any border crossing - you have to submit to searches by CBP agents, who don't need any probable cause. I believe device searches are still a somewhat greyer area, at least if password protected (as providing a password would be compelled speech, unlike a luggage search which can be conducted without any action from your part).
in autistic-land, doesn't it seem like the power to perform searches by border police should be directed related and limited to their function - that is the inspection of material goods to insure that (a) illegal smuggling isn't taking place and (b) the proper import taxes have been paid. unless the phone itself is a suspicious good, there should never be any justification for a border officer to demand visibility of…
Personally, I believe that there should be explicit laws protecting your personal devices, given the amount of sensitive private information they carry. I think an explicit law, or even amendment to the constitution, would be much better than relying on ever more arcane legal interpretations being invented by judges based on vague wording and vague principles.