alt Hacker Newsin 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 the contents of the phone, or the data that its permitted to access on other systems.
I presume the counter to this is that they must be able to examine the socials in order to see if you should 'death to america' on message boards. is that really an important vector? is the balance of that security concern versus speech rights (which are supposed to be 'universal') something we want individual field officers to decide?
I think one of their main justifications for claiming a right to inspect your phone is the possibility that you could be carrying illegal data on it, such as CSAM (which they apparently invoked in this particular case). Since they generally can inspect your luggage to check if you're crossing the border with illegal goods, this seems somewhat plausible as a legal interpretation.
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.