The 5-to-3 decision amounted to a green light for vigorous state efforts to combat the employment of illegal workers. The majority opinion, written by Chief Justice John G. Roberts on behalf of the court’s five more conservative members, noted that Colorado, Mississippi, Missouri, Pennsylvania, Tennessee, Virginia and West Virginia had recently enacted laws similar to the one at issue in the case.
(I'll get to why it's not really a reasonable decision in a second...)
The onus of the hiring of undocumented workers clearly rests with the employer. After all, the immigrant is not creating jobs out of whole cloth (and if they can create jobs, they ought to be fast-tracked to citizenship.) There needs to be someone here who is willing to offer a job to a non-American worker.
This is precisely where the focus of enforcement lies, or rather should lie, instead of building fences and patrolling with dogs and shotguns. If there are no jobs to be had for undocumented workers, they won't cross the border. It's really that simple.
Now, the Obama administration had joined with business and civil liberties groups in opposition to this law, and the focus of that is on a fairly important point: this state law supercedes a 1986 Federal law on immigration, and the Constitution specifically reserves "states' rights" for issues that the Federal cannot or will not address.
Which is not the case here. The SCOTUS has made a fairly stupid decision on that point, one I expect will be overturned by a future SCOTUS. Preferably soon.