I agree with you that it's patently illegal, but most likely ice will arrest decliners for 48 hours and not charge them with anything. It is a crime to not identify yourself to officers, but you're ONLY required to provide a name (or license if driving). They would not be able to get an obstruction charge to stick for facial scan. Just remember not to resist the arrest itself, and definitely don't fight back since then they can get you on resisting or battery leo. Just look down, close your eyes, anything you can do to interfere with the scan.