The Supreme Court struck down most of Arizona's immigration law on Monday. "The court ruled that Arizona cannot make it a misdemeanor for immigrants to fail to carry identification that says whether they are in the United States legally; cannot make it a crime for undocumented immigrants to apply for a job and cannot arrest someone based solely on the suspicion that the person is in this country illegally," my Washington Post colleagues report.

But the justices upheld one of the most controversial parts of the law, which requires state law enforcement officials to check the immigration status of anyone they stop, detain or arrest for another offense and also suspect to be an illegal immigrant — what immigration advocates have dubbed the "papers, please" provision.

Arizona immigration supreme court sb1070 A supporter of the federal challenge to Arizona's anti-immigration law stood outside the U.S. Supreme Court in April as the justices were hearing oral arguments in the lawsuit. (Washington Post/Matt McClain)

The mixed decision partly explains why media outlets have had such wildly divergent takes on the high court's ruling: CNN and MSNBC emphasized that major parts of the law were struck down, while Fox trumpeted that some parts were upheld, as BuzzFeed's Andrew Kaczynski points out. But Fox might not want to break out the champagne quite yet: The Supreme Court explicitly left the door open for subsequent challenges to the "papers, please" provisions.

"This opinion does not foreclose other preemption and constitutional challenges to the law as interpreted and applied after it goes into effect," the court explained in its decision.

In other words, it will essentially depend on how state law enforcement actually carries out the law — which was enjoined in Arizona before it was ever put into effect—and how state courts rule on any future legal challenges to it. As it stands, "there is a basic uncertainty about what the law means and how it will be enforced," the Supreme Court explained. The decision said that the provision "could be read to avoid" violating the Fourth Amendment and that it's "likely" to avoid problems with preempting federal law. And it adds that "at this stage, without the benefit of a definitive interpretation from the state courts, it would be inappropriate" to assume otherwise. So while the court ruled that part of the law is constitutional in theory, its decision implies that it might not be constitutional in practice. And only time will tell.

That's why policy experts like Ben Winograd, a staff attorney for the American Immigration Council, believe the provision remains "legally vulnerable": It could still be struck down if it leads to civil rights violations, as pro-immigration advocates have warned. The same could happen in the five other states that have passed Arizona-like provisions for checking immigration status: Alabama, Utah, South Carolina, Indiana and Georgia. "They upheld the racial profiling provision, 2(B), but only on the narrowest grounds," says Greg Chen of the American Immigration Lawyers Association.

Winograd, for one, expects problems to mount up quickly for the provision. "It comes down to the assumption on whether the police respects the Constitution on a day-to-day basis," he says. "On paper, it makes a lot of sense. In practice, it's hard to believe."