The crux of the matter is here:
More importantly, travelers are not legally required to unlock their devices, although agents can detain them for significant periods of time if they do not....
and here: The document given to Bikkannavar listed a series of consequences for failure to offer information that would allow CBP to copy the contents of the device. “I didn’t really want to explore all those consequences,” he says. “It mentioned detention and seizure.”
It sounds like CBP is trying to circumvent the "PIN Revealing" need by basically illegally detaining Citizens until they do.
This is grounds for "Habeas Corpus" lawsuit - should a citizen really dig their heels in.