Something that tends to get lost is that being an undocumented emigrant is a civil offense, not a criminal one. So, local officials are supposed to keep people in custody for however long it takes ICE to show up? Are you ready to pay the legal costs of your local officials when they get sued for depriving people of their freedom? (It has happened) If the undocumented person committed a criminal offense, the local officials can only hold them until their sentence is served. Holding them after that becomes very problematic.