Only because it has not been implemented yet, and because SCOTUS was uncertain about what it meant and how it would be enforced.By the way, this is the part of the law SCOTUS left standing.
The SCOTUS is already propped to throw that one out also, when AZ implements it, based on their comment regarding it.
"The nature and timing of this case counsel caution in evaluating the validity of [Section] 2(B)," wrote Justice Anthony Kennedy on behalf of Chief Justice John Roberts and Justices Ruth Bader Ginsburg, Stephen Breyer and Sonia Sotomayor, noting that the law has not yet gone into effect. Because "[t]here is a basic uncertainty about what the law means and how it will be enforced," the majority chose to allow the law to go forward, but made clear that "[t]his opinion does not foreclose other preemption and constitutional challenges to the law as interpreted and applied after it goes into effect."
Arizona Immigration Law Ruling: Supreme Court Delivers Split Decision