By Cherri Gregg

PHILADELPHIA (CBS) — The US Supreme Court surprised the nation yesterday when it upheld President Obama’s health care law.

We spoke to a constitutional law expert to explore some of the logic behind the court’s ruling.

Read Cherri Gregg’s “Health Care Overhaul FAQ”

Scholars who believed that Justice Kennedy would decide the fate of the Affordable Care Act were shocked when Chief Justice John Roberts turned out to be the swing vote.

But some are now speculating that Roberts originally voted to strike down the individual mandate, then changed his mind in the final days before the decision was handed down.

“It may well be that as the chief justice, John Roberts cared a little more about the court’s institutional posture and just was not willing to take what would have been an extraordinarily activist step,” says Ted Ruger, a law professor at the University of Pennsylvania and former clerk to Justice Stephen Breyer.

Ruger says that instead of striking down the law, Roberts found a way to uphold it.

“Justice Roberts, who clearly wasn’t comfortable justifying the mandate under the general power of the Commerce clause, found a loophole, if you will, because he’s right: the mandate is something that is administered as a tax. It’s something that people would pay on their tax returns.  No one will put in jail over this.”

Roberts writes in his opinion that it was not the court’s job to decide the wisdom of laws, only their constitutionality.  The rest, he said, is left to elected officials.