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.