Unfortunatly section 6 gives AZ the power to determine what are deportable offenses, and infringes on the power of the federal government. Which is why it’s been struck down.
The Federal government has sole authority over deportation. The section was not struck down for the reason you cited,
Section 6 authorizes officers to arrest without a warrant a person “the officer has probable cause to believe . . . has committed any public offense that makes the person removable from the United States.” §13–3883(A)(5). The United States argues that arrestsauthorized by this statute would be an obstacle to the removal system Congress created.
As a general rule, it is not a crime for a removable alien to remain present in the United States. See INS v. Lopez16 ARIZONA v. UNITED STATES Opinion of the Court Mendoza, 468 U. S. 1032, 1038 (1984). If the police stop someone based on nothing more than possible removability, the usual predicate for an arrest is absent. When an alien is suspected of being removable, a federal official issues an administrative document called a Notice to Appear. See 8 U. S. C. §1229(a); 8 CFR §239.1(a) (2012).The form does not authorize an arrest. Instead, it givesthe alien information about the proceedings, including the time and date of the removal hearing. See 8 U. S. C. §1229(a)(1). If an alien fails to appear, an in absentia order may direct removal. §1229a(5)(A).
>The federal statutory structure instructs when it is appropriate to arrest an alien during the removal process. For example, the Attorney General can exercise discretion to issue a warrant for an alien’s arrest and detention “pending a decision on whether the alien is to be removedfrom the United States.” 8 U. S. C. §1226(a); see Memorandum from John Morton, Director, ICE, to All Field Office Directors et al., Exercising Prosecutorial Discretion Consistent with the Civil Immigration Enforcement Priorities of the Agency for the Apprehension, Detention, and Removal of Aliens (June 17, 2011) (hereinafter 2011 ICEMemorandum) (describing factors informing this and related decisions). And if an alien is ordered removed after a hearing, the Attorney General will issue a warrant. See 8 CFR §241.2(a)(1). In both instances, the warrants are executed by federal officers who have received training in the enforcement of immigration law. See §§241.2(b), 287.5(e)(3). If no federal warrant has been issued, those officers have more limited authority. See 8 U. S. C. §1357(a). They may arrest an alien for being “in the United States in violation of any [immigration] law or regula- tion,” for example, but only where the alien “is likely toescape before a warrant can be obtained.” §1357(a)(2).
Section 6 attempts to provide state officers even greater ability than Congress has given to trained federal immigration officers. Under state law, officers who believe an alien is removable by reason of some “public offense” would have the power to conduct an arrest on that basis regardless of whether a federal warrant has issued or the alien is likely to escape. This state authority could be exercised without any input from the Federal Government about whether an arrest is warranted in a particular case. This would allow the State to achieve its own immigration policy. The result could be unnecessary harassmentof some aliens (for instance, a veteran, college student, or someone assisting with a criminal investigation) whom federal officials determine should not be removed.
This is not the system Congress created. Federal law specifies limited circumstances in which state officers may perform the functions of an immigration officer. A principal example is when the Attorney General has granted that authority to specific officers in a formal agreement with a state or local government. See §1357(g)(1); see also§1103(a)(10) (authority may be extended in the event of an“imminent mass influx of aliens off the coast of the United States”); §1252c (authority to arrest in specific circumstance after consultation with the Federal Government);§1324(c) (authority to arrest for bringing in and harboring certain aliens). Officers covered by these agreements aresubject to the Attorney General’s direction and supervision. §1357(g)(3). There are significant complexitiesinvolved in enforcing federal immigration law, including the determination whether a person is removable. See Padilla v. Kentucky, 559 U. S. ___, ___–___ (2010) (ALITO, J., concurring in judgment) (slip op., at 4–7). As a result, the agreements reached with the Attorney General must contain written certification that officers have received adequate training to carry out the duties of an immigration officer. See §1357(g)(2); cf. 8 CFR §§287.5(c) (arrest power contingent on training), 287.1(g)
FYI: Today, the Obama administration pulled 287 g authority away from AZ. They did it because they did not like the SCOTUS ruling. In essence, they have taken away AZ's ability to enforce immigration laws even with trained personnel. This is a travesty and is dispicable.