Arizona is taking up the task of enforcing immigration law because the feds are deliberately avoiding their duty to do so.
The problem here is not Arizona. The problem is a federal government that has abandoned the rule of law.
What legitimacy can a government have when it not only fails to enforce its laws consistently, but refuses to enforce some laws altogether?
The legitimacy of any state is derived from its people, not the other way around. This is something that many Americans have forgotten. Many Americans seem to think that the government is there to tell us what to do and that we have no choice but to obey. Government officials are not our masters. They are not feudal lords and we are not peasants and vassals. They have no authority except that which defined by law. Authority that carries with it the obligation to enforce that law.
The sole purpose of state power is to enforce the law. The powers of the state are defined, and limited, by law. The authority to enforce the law carries with it the OBLIGATION to do the same. Just as the state cannot assume powers not defined by law, neither can it neglect to enforce the law using the powers it has been given.
Any government that neglects its duty to enforce the law as written is illegitimate. Any government official who acts in bad faith and shirks his or her duty to enforce the law as written has violated their oath of office and should be subject to civil, if not criminal, prosecution.