Why Roberts joined the Centrist Four

This is apparently what CBS news reporter Jan Crawford learned from anonymous court insiders about CJ Roberts's reasoning in leading a unique majority to uphold the ACA.

Some informed observers outside the court flatly reject the idea that Roberts buckled to liberal pressure, or was stared down by the president. They instead believe that Roberts realized the historical consequences of a ruling striking down the landmark health care law. There was no doctrinal background for the Court to fall back on - nothing in prior Supreme Court cases - to say the individual mandate crossed a constitutional line. The case raised entirely new issues of power. Never before had Congress tried to force Americans to buy a private product; as a result, never before had the court ruled Congress lacked that power. It was completely uncharted waters.
To strike down the mandate as exceeding the Commerce Clause, the court would have to craft a new theory, which could have opened it up to criticism that it reached out to declare the president’ health care law unconstitutional.
Roberts was willing to draw that line, but in a way that decided future cases, and not the massive health care case.

To me, this seems much more plausible and likely than any of the Right Wing conspiracy theories as to why they were "betrayed," or the assumption that Roberts was concerned about the "legitimacy" of the court, although that factor, in my opinion, should have been in the forefront of his mind, and perhaps it was; we can't really know.

Of course, allowing the Right to frame health care as a "product," rather than as an essential service in the provision of which government has been intimately involved for many many decades, is part of the problem; and in this regard I believe the government and the government's lawyers have done a poor job of defending the law both before the court and in the court of public opinion. Our party leaders, and especially the president himself, need to do a much better job of explaining how the act benefits most Americans, and in reframing the whole debate in terms of progressive moral stances, rather than Rightist economic ones.