Despite this thread's title, DHS's own Office of Civil Rights is not a "watchdog" in any conventional meaning of the term. Its relationship to the Dept. of Homeland Security is akin to the Inquisition's relationship to the Holy See. Thus its policy, which that it's OK for CBP to seize any electronic gadget being carried into the US by any traveller, citizen or not, and keep it for an indeterminate "reasonable" amount of time, should not be assumed to be the final word on the 4th Amendment.
For your reading pleasure, here is a guide for travelers entering the US who are interested in retaining electronic privacy:
https://www.eff.org/sites/default/fi...r-search_2.pdf Note that the border guards cannot force you to divulge device passwords. A court could order you to provide the passwords, assuming you weren't successful in arguing your 5th Amendment self-incrimination right, but the penalty for refusing this order would be prison, not the rack (at least in theory, we haven't yet stooped to waterboarding civilian citizens).
[Note my alternating use of traveler/traveller -- I was educated in the US, Australia and Canada and spell as my mood suits me.]