Reader Stuart Buck provides more detail as to why the dissent reads like a majority opinion (see also Deborah Pearlstein at Balkanization):
1. The dissent has a whole section on severability that is completely beside the point except on the assumption that the mandate had been struck down, and now “We” have to decide whether and what to preserve of the rest of the act now that the mandate is gone.
2. Notice also that his response to Roberts is tacked on at the end, rather than worked into the body of whatever he was writing (see page 64 of his dissent). For example, one would have expected Scalia to directly take on Roberts’ application of the Anti-Injunction Act, but his brief section on that act only mentions what “the Government” argues (see pages 26-28).
3. On top of that, Scalia’s sections on the Commerce Clause and the Medicaid Expansion are just as long or longer than what Roberts writes (Scalia wrote 16 pages on the Commerce Clause and 21 pages on the Medicaid Expansion, compared to Roberts’ 16 pages and 14 pages respectively). Yet Scalia never writes in the vein of saying, “I agree with the Chief Justice’s opinion, but write to add a crucial discussion of some complexity.” His analysis agrees with Roberts, and makes essentially the same points in “We” language. There’s no reason for Scalia to do this at such length, unless his opinion is what came first.
UPDATE: Ed Whelan notes a related theory: Roberts assigned the opinion to himself, and wrote most of what became the four-Justice dissent. He then switched on the tax issue, and the four dissenters adopted most of his original majority opinion as a dissent. This would explain why the dissent is unsigned. Other blogs are noting that Justice Ginsburg directs much of her ire at the Chief, which is the sort of things Justices do when they think they’ve lost someone’s vote, not when they are trying to keep a tenuous vote to uphold uphold the law in question on board.
I should note that I think the Supreme Court is a political body (which is not to say that its decisions are primarily motivated by partisanship or political ideology) and that one can expect that the Court’s rulings are affected by outside events. As I noted long ago, the challenge to the individual mandate would have stood no chance if the president and the ACA were riding very high in the polls, as the Court would not have had the political wherewithal to write what would be seen as a radical opinion invalidating a popular law from a popular president. Similarly, the level of heat defenders of the ACA were giving the Court could have persuaded Roberts that discretion was the better part of valor. (By contrast, for example, historians seem to think that FDR was relieved that the Supremes removed the NIRA albatross from his neck.) Perhaps, as Rick Hasen suggests, he’d rather save his political capital for the affirmative action and voting rights cases that are coming up, especially since he found a way to give the “right” a partial victory in his commerce clause reasoning, and to limit the Spending power.
I don’t find it at all illegitimate for political actors to put pressure on the Court, so long as they stay within proper legal bounds, and keep their rhetoric within the broad boundaries of decency. But it is ironic that while liberal critics were quick to accuse the Court of playing politics by taking seriously the Obamacare challenges, it may turn out that it was only politics that saved the ACA.