The nitty-gritty of the legal argument is this. When Congress passed the Immigration Reform & Control Act, it expressly pre-empted ""any State or local law imposing civil or criminal sanctions (other than through licensing and similar laws) upon those who employ . . . unauthorized aliens."
It is the parenthetical phrase "(other than through licensing and similar laws)" that the Arizona legislature has driven the proverbial truck through, and now the Supreme Court has backed them up.
I will leave it to the immigration experts to talk about the impact on that particular body of law. The politico's can talk about what will happen from a political standpoint. My amateur observation is that many states, including Texas, will pass similar laws, a move will be made in Congress to roll back the savings clause, and all of these actions will be more for political purposes than for resolution of a national problem.
From a positive perspective, it is possible given that the defendant in this case was the U.S. Chamber of Commerce that this will be the spark (or explosion) that leads to serious discussions to come up with a rational national solution. (And I say to myself, and pigs may fly.)
As a textbook statement of what the world should be like, I would not quarrel with the logic of Justice Robert's statement.The Chamber and JUSTICE BREYER assert that employers will err on the side of discrimination rather than risk the “‘business death penalty’” by “hiring unauthorized workers.” [cites omitted] That is not the choice. License termination is not an available sanction simply for “hiring unauthorized workers.” Only far more egregious violations of the law trigger that consequence. The Arizona law covers only knowing or intentional violations. The law’s permanent licensing sanctions do not come into play until a second knowing or intentional violation at the same business location, and only if the second violation occurs while the employer is still on probation for the first. These limits ensure that licensing sanctions are imposed only when an employer’s conduct fully justifies them. An employer acting in good faith need have no fear of the sanctions.