The second-silliest reactions coming from today's ACA opinion are the conservatives comparing Bush II's nomination of Roberts to Bush I's nomination of Souter. This is hardly fair. Roberts has been a sound fifth conservative vote on critically important First and Second Amendment issues; he's consistently refused to abuse the Eighth Amendment to strike down legitimate exercises of state legislative power in criminal law; he's consistently enforced Congress's limitations on habeas relief. And today, he signed onto both the broadest restrictions of Congress's Commerce Clause power in decades and the first teeth in South Dakota v. Dole, limiting the ability of the federal government to bully the states. (The silliest reaction? The retroactive wishes for Justice Harriet Miers—which would be objectively silly even if it wasn't for the historical fact that Miers was nominated for Alito's seat, not Roberts's.)
One can be dismayed about the broad scope of the taxing power implicated by today's decision, but that is not anything new; for example, you've been paying extra taxes for failing to buy an electric car since at least the 2001 tax year, and extra taxes for not having a residential mortgage for even longer. (These are called tax credits, rather than penalties or taxes, but they're economically indistinguishable at the margin or otherwise, somewhat refuting Richard Epstein's complaint.)
The complaint is perhaps whether the "penalty" should be called a "tax" when Congress refused to call it a "tax"; the dissent would hold Congress to its language, while Roberts, alone, looks purely at the economics of the matter. Both arguments are colorable: after all, the Court has previously characterized "taxes" as "penalties" when they held the character of penalties, so why not vice versa? To which the Scalia dissent responds that this is the first time the Court has done so, and it is the finest of hair-splitting to say that a penalty isn't a tax for purposes of the Anti-Injunction Act, but is for purposes of the Taxing Power inquiry.
I've previously been unhappy with Roberts's tendencies to blue-line rewrite statutes to avoid tough constitutional questions; the canon of constitutional avoidance is one thing, but creating non-existent text to fix problems just seems to me outside the Article III power. We saw this in Free Enterprise Fund, NAMUDNO, and Wisconsin Right to Life. With it happening again today both in the construction of the penalty as a tax and the rewrite of the Medicaid penalties to the states, we can officially note an unhappy trend in the Chief Justice Roberts jurisprudence.
ACA opponents have an out in the Roberts opinion: it remains prohibited for the taxing power to be excessively punitive, a matter not well raised in the briefs. "Because the tax at hand is within even those strict limits, we need not here decide the precise point at which an exaction becomes so punitive that the taxing power does not authorize it" (slip op. 43). But ACA imposes marginal income "taxes" of over 100% on certain members of the middle class who are in a particular donut-hole of income. Expect to see a new challenge in the future on this, and on other aspects of ACA.
Update: Typos in the Scalia dissent—which repeatedly refer to the "Ginsburg dissent"—show that it was originally meant as a majority opinion? [DeLong; see also Bernstein @ Volokh] One hopes very much that the Roberts flip was a sincere decision consistent with his previous overbroad canon of constitutional avoidance, rather than a "switch in time to save nine" prompted by the offensive degree of lobbying and attacks on the Court's integrity by the Obama administration and its allies.