Amid the controversy arising from the federal district court's decision to strike down portions of Arizona's Senate Bill 1070, one must keep in mind the fact that the case is at its most preliminary stage. Judge Bolton, of the U.S. District Court for the District of Arizona, did not issue a final decision on the merits of the case; she merely granted the federal government a "preliminary injunction," preventing Arizona from enforcing portions of SB 1070 until the court undertakes full review of the case and issues a final decision on the merits. Accordingly, the court did not hold that portions of SB 1070 are unconstitutional; it held merely that the "the United States is likely to succeed on the merits" when the case is heard in full.
Of course, that's probably a distinction without a difference. Judge Bolton likely will not reverse course and rule in Arizona's favor, unless either the U.S. Court of Appeals for the Ninth Circuit or the U.S. Supreme Court rejects the district court's analysis of federal law's effect on SB 1070. Arizona already has appealed to the Ninth Circuit, and the oral argument is scheduled for November.
Nevertheless, the district court's decision already has been the subject of harshly cogent criticism by Heather Mac Donald, Andy McCarthy, and former assistant secretary of homeland security for policy Stewart Baker, among others. But with respect to the most controversial aspect of the court's decision – striking down SB 1070's mandate to check the immigration status or lawful presence of certain persons – a few points deserve amplification.
SB 1070's immigration-verification provision is comprised of two controversial requirements:
 For any lawful stop, detention or arrest ... where reasonable suspicion exists that the person is an alien and is unlawfully present in the United States, a reasonable attempt shall be made, when practicable, to determine the immigration status of the person, except if the the determination may hinder or obstruct an investigation.  Any person who is arrested shall have the person's immigration status determined before the person is released. The person's immigration status shall be verified with the federal government pursuant to 8 [U.S.C.] 1373(c).
8 U.S.C. 1373(c), in turn, is a federal statute that requires the federal Immigration and Naturalization Service to "respond to an inquiry by a Federal, State, or local government agency, seeking to verify or ascertain the citizenship or immigration status of any individual within the jurisdiction of the agency for any purpose authorized by law, by providing the requested verification or status information."
In her decision, Judge Bolton reviewed SB 1070's second provision first, and interpreted it in the strictest manner possible: She concluded that by using the term "any person who is arrested," SB 1070 requires law enforcement to contact INS with respect to every single person who is arrested for any reason whatsoever. Then, the court concluded that involving INS with every single arrest in the state of Arizona would put an impermissible burden on INS's resources; Judge Bolton held that federal law preempted that provision.