They can absolutely keep them. Or they can just "lose" them "accidentally". Who exactly would force them to give them back? Or put another way, who exactly will punish them if they break the law?
Morality and direct commonsense interpretations of law do not apply when there are literally unlimited resources stacked against you. But, assuming you can wait the potential ~10y to receive your device back that it will take to get your device returned to you, good on you. If you think that the current SCOTUS will rule in your favor, good on you.
The reality is, we live in a time where the most horrendous interpretation of the law is the one that will happen. And it won’t be in your favor.
My bad.
If they do take my phone (completely shut down, unlikely they'll be able to break in) and it's gone forever, that sucks, but then I get a new phone, restore from a backup, and move on with my life. Given that the probability of getting to this point is very low, I'm comfortable with the risk.
But sure, if I was at high risk of being detained at the border due to my profession, country of origin, ethnicity, etc., I'd probably look at this differently.