CBP has broad search authority at borders without warrants, and while they can't criminally charge you for refusing to unlock your device, they can deny entry,

CBP has broad search authority at borders without warrants, and while they can't criminally charge you for refusing to unlock your device, they can deny entry, seize the device, and ban you from future entry - effectively making the "5 years in prison" threat misleading but the practical consequences still severe.