The 5-4 language suggests that Roberts agreed with the liberals. But for the most part, he didn’t. If you read the opinions, he sided with the conservative bloc on every major legal question before the court. He voted with the conservatives to say the Commerce Clause did not justify the individual mandate. He voted with the conservatives to say the Necessary and Proper Clause did not justify the mandate. He voted with the conservatives to limit the federal government’s power to force states to carry out the planned expansion of Medicaid. ”He was on-board with the basic challenge,” said Orin Kerr, a law professor at George Washington University and a former clerk to Justice Kennedy. “He was on the conservative side of the controversial issues.”
His break with the conservatives, and his only point of agreement with the liberals, was in finding that the mandate was a “tax” — a finding that, while extremely important for the future of the Affordable Care Act, is not a hugely consequential legal question.
So basically, most of the court bought the inactivity and broccoli arguments. They thought that congress can't regulate something people aren't doing, which was not buying health insurance. And they thought that if congress can force you to buy health insurance, they could force you to do a lot of other things they don't like. I'll direct you to this post for most of my thoughts on those arguments.
But Chief Justice Roberts thought that the mandate was really just a tax. And congress clearly has the power to tax. (And apparently, it can tax whatever it wants.) So based on that reasoning, Roberts voted with the liberals to uphold the entire bill.
I was a bit surprised by the ruling. I thought it would be overturned. And I thought Kennedy would be the swing vote whichever way it turned out. Even though I'm disappointed with the commerce clause ruling and the medicaid part that Roberts ruled against, I'm glad Roberts was reasonable and upheld the bill. This isn't the silver bullet for our health care problems. And given electoral outcomes, Romney and Republicans could repeal the bill. But I think that will be difficult for them to do that. And with a little luck this could make a big positive difference for this country.