That's the point, "constitutionally" while they lock you up for hours/days on end to obtain the warrant needed to give up your password unless you are willing to stay locked up.
Many border agents, it seems, have the “if you have nothing to hide” mentality. So if you’re refusing to unlock your phone, clearly you’re hiding something.
Get that sweet settlement for wrongful imprisonment.
How many times has this actually happened though?
Or, you know, no settlement, and not even a "sorry, oops" either.
That would be comforting if both (a) and (b) were true:
(a) the world was only US citizens.
(b) nothing unconstitutional ever happened to the first group.