U.S. District Court Judge Susan Bolton essentially enjoined the State of Arizona from implementing the most controversial part of AZ 1070 — section 11-1051(B) — dealing with reasonable suspicion stops by law enforcement based on the "reasonable suspicion a person is an alien." This section, along with a few others, have been enjoined. The remainder of the law, which contains provisions more within the realm of state regulatory control and lawmaking, is unaffected.
It is well documented in court decisions that immigration enforcement and regulation of immigration is a federal matter. Two prior U.S. Supreme Court cases provide guidance on the issue: U.S. v. Brignoni-Ponce, 422 U.S. 873 (1975) which held that police officers may not arrest individuals for immigration enforcement purposes except upon probable cause and DeCanas v. Bica, 424 U.S. 351 (1976) which held any local law, rule, statute which seeks to regulate immigration violates the Supremacy Clause.
State involvement in this area, except as specifically allowed under federal law (such as 287g programs), is preempted by the Supremacy Clause to the U.S. Constitution. The crux of the judge's decision rests on the burden the law would place on legal resident aliens who may be unconstitutionally held as a result of this law, and signifies (at least partly) the judge's belief in the federal government's success on the merits of its argument against the State of Arizona on those provisions of the law she disallowed.
What this means for police officers in Arizona is that those whose instincts regarding enforcement of the law gave them pause and created concern about the impact of their enforcement of this controversial law were correct. Every police officer in America is well aware of the scope of Terry v. Ohio regarding stops based on reasonable suspicion — that reasonable suspicion be based on a belief a crime has been committed or is about to be committed. The Arizona law places additional criteria for reasonable suspicion stops based on status.
It is interesting to note that the judge left alone the portion of 11-501 which dealt with aliens who have been convicted of a crime and are being released upon completion of sentence being held for the federal authorities. Clearly, this is an acceptable practice and does not have the same 4th Amendment implications of a stop on the street.