Americans 4th Amendment right wasn't created by the 4th Amendment, but merely acknowledged by it. All human beings have an inherent right to privacy -- to suggest otherwise as a US citizen is self-contradictory.
And more simply, it means you have to trust border patrol's password management as well as your own. (Or change them all immediately.)
For example:
UNCLOS - "Although the United States helped shape the Convention and its subsequent revisions, and though it signed the 1994 Agreement on Implementation, it has not signed the Convention as it objected to Part XI of the Convention." https://en.wikipedia.org/wiki/List_of_parties_to_the_United_...
UN Convention on the Rights of the Child: "The United States government played an active role in the drafting of the Convention and signed it on 16 February 1995, but has not ratified it" https://en.wikipedia.org/wiki/Convention_on_the_Rights_of_th...
Basel Convention: "Haiti and the United States have signed the Convention but not ratified it." https://en.wikipedia.org/wiki/Basel_Convention
Additionally, the US wages illegal wars all the time - against the UN charta where only aggression for self defence or with a UN mandate is allowed.
"When he went through secondary inspection at Vancouver airport, US Customs officers didn’t even need to ask for his passwords — they were saved in their own system."
"They said, ‘Next time you come through, don’t have a cleared phone,’ and that was it. I wasn’t let through. "
--
I know that US/Canada privacy laws might not give people protection in this situations, but what about universal human rights? UN Declaration of Human Rights and other binding international agreements recognize the right to privacy.