Justices Scalia and Thomas would have supported the Arizona law making it a crime for non-citizens to be without legal immigration papers. Immigration law is complex and filled with opportunities for unpredictable judge discretion. Many undocumented immigrants, once they get before a judge, discover they have a legal basis for being in the U.S. In our innocent-until-proven-guilty legal system, such immigrants have committed no crimes. The Arizona law not only assumed guilty-until-proven-innocent, it actually created the crime of not having previously proven oneself innocent of another crime. Thankfully, such state laws are now clearly unconstitutional.
— Rundown of losing legal theories in Arizona v. United States. I found this one particularly poignant — I knew the AZ law went in the highly undesirable direction of a “Papers, please” state, but had not yet thought of it in quite this light.