I suggest a simple answer - Roberts is a LIBERAL.
The problem is, if Roberts is a liberal, then EVERY Supreme Court Justice is a liberal. If you read all of the decisions, you’ll see that EVERY Justice (even Scalia, Thomas. Alito and Kennedy) agreed that Congress would have had the power to impose a tax on people who did not buy health insurance. Where the majority and dissent disagreed is that the majority construed Obamacare as a tax, while the dissent did not. That’s a huge difference of statutory interpretation, of course, and it led to a dreadful result, but, in terms of the expansion of Congressional taxation power, there was no difference between any of the Justices.