So far they are, for citizens. The worst case I've heard of was a citizen who was held by ICE for a few hours because they didn't believe him. Which, don't get me wrong, is bad given the racist underpinnings of it, but not something that worries me in and of itself. And yeah, they've gone so far as to deport US citizens from time to time over the years, but it's not anything like systematic.
They're not refusing entry or holding citizens indefinitely for refusing to unlock their phones at the border. Maybe they will, but until it starts, what I said holds.