See: Why Arizona Drew a Line
State governments aren’t allowed to get involved in immigration, which is a federal matter. While it is true that Washington holds primary authority in immigration, the Supreme Court since 1976 has recognized that states may enact laws to discourage illegal immigration without being pre-empted by federal law. As long as Congress hasn’t expressly forbidden the state law in question, the statute doesn’t conflict with federal law and Congress has not displaced all state laws from the field, it is permitted. That’s why Arizona’s 2007 law making it illegal to knowingly employ unauthorized aliens was sustained by the United States Court of Appeals for the Ninth Circuit.
This Op-Ed is a good resource to use as this subject comes up around the water-cooler.
HT: (UFO TOFO, See #108) at Correspondence Committee