Assume for a moment that Roberts actually believes that the individual mandate is constitutional. Crazy, I know, but bear with me for the moment.
Remember, Roberts could've taken a much smoother path to upholding the whole law, but he didn't. He could've concluded that respected, conservative circuit court judges Jeff Sutton and Laurence Silberman had it right when they ruled on the constitutionality of 'Obamacare,' and upheld the law as a valid exercise of the Commerce Clause.
Instead he rejected the Sutton/Silberman argument and took the more circuitous route -- a route that preserved his ability to cabin the Commerce Clause in the way conservatives wanted. Now I don't particularly want to ascribe motives to Roberts. But if like Mitt Romney and other conservatives you're willing to assume Roberts wrote an opinion he doesn't believe, you can just as easily assume he believes the mandate is a valid exercise of Commerce Clause powers but chose to argue otherwise, in order to narrow federal power and throw his ideological allies a bone.
There's a third possibility -- that Roberts believes every word of what he wrote. Again, crazy, right? But many of Roberts' erstwhile allies have been happy to posit ulterior motives. The point I'm making is that the ulterior motives could cut either direction.
We do know that Roberts believed it was specifically important for his Commerce Clause argument to have precedential value. Justice Ginsberg argued in a dissenting opinion that having upheld the mandate on the basis of a different constitutional power, the Court had no obligation to superfluously limit the Commerce Clause.
Roberts rebuffed her by more or less admitting he had to strain his reading of the mandate to uphold it as a taxing power exercise.
"Justice Ginsburg questions the necessity of rejecting the Government's commerce power argument, given that [the mandate] can be upheld under the taxing power," Roberts wrote. "But the statute reads more naturally as a command to buy insurance than as a tax, and I would uphold it as a command if the Constitution allowed it. It is only because the Commerce Clause does not authorize such a command that it is necessary to reach the taxing power question. And it is only because we have a duty to construe a statute to save it, if fairly possible, that [the mandate] can be interpreted as a tax. Without deciding the Commerce Clause question, I would find no basis to adopt such a saving construction."
If he'd reached the taxing power conclusion in a straightforward manner, he would have foreclosed on his ability to circumscribe the Commerce Clause in the way he did. Conservatives construe his less-straightforward argument as evidence of politicization, but it is just as plausibly evidence of his desire to wring a consolation prize for conservatives out of a losing health care case.