The mandate opinions are here. The Chief’s opinion is the main one, even though he doesn’t have a majority on all parts. Notably, Roberts begins with nice discussion of the role of federalism and the courts, presumably intended for the public audience that will read the opinion.
On the mandate, the Chief then goes on to agree with Randy Barnett’s activity/inactivity theory. He writes:
To an economist, perhaps, there is no difference between activity and inactivity; both have measurable economic effects on commerce. But the distinction between doing something and doing nothing would not have been lost on the Framers, who were “practical statesmen,” not metaphysical philosophers. Industrial Union Dept., AFL–CIO v. American Petroleum Institute, 448 U. S. 607, 673 (1980) (Rehnquist, J., concurring in judgment). As we have explained, “the framers of the Constitution were not merevisionaries, toying with speculations or theories, but practical men, dealing with the facts of political life as they understood them, putting into form the government they were creating, and prescribing in language clear and intelligible the powers that government was to take.” South Carolina v. United States, 199 U. S. 437, 449 (1905). The Framers gave Congress the power to regulate commerce, not to compel it, and for over 200 years both our decisions and Congress’s actions have reflected this understanding. There is no reason to depart from that understanding now.
Roberts also construes the Necessary and Proper Clause narrowly — construing it to bring inactivity in the scope of the Commerce Clause would not be “proper”:
[S]uch a conception of the Necessary and Proper Clause would work a substantial expansion of federal authority. No longer would Congress be limited toregulating under the Commerce Clause those who by some preexisting activity bring themselves within the sphere of federal regulation. Instead, Congress could reach beyond the natural limit of its authority and draw within its regulatory scope those who otherwise would be outside of it. Even if the individual mandate is “necessary” to the Act’s insurance reforms, such an expansion of federalpower is not a “proper” means for making those reforms effective.
But Roberts then goes on to rule on the tax power. The key step, it seems to me, is the standard for determining what counts as a tax. Roberts argues that precedents require the Court to look for any reasonable construction of the statute to bring it within the tax power:
The question is not whether that is the most natural interpretation of the mandate, but only whether it is a “fairly possible” one. Crowell v. Benson, 285 U. S. 22, 62 (1932). As we have explained, “every reasonable construction must be resorted to, in order to save a statute from unconstitutionality.” Hooper v. California, 155 U. S. 648, 657 (1895). The Government asks us to interpret the mandate as imposing a tax, if it would otherwise violate the Constitution. Granting the Act the full measure of deference owed to federal statutes, it can be so read[.]
That is true because the mandate in operation acts much like a tax:
First, for most Americans the amount due will be far less than the price of insurance, and, by statute, it can never be more. It may often be a reasonable financial decision to make the payment rather than purchase insurance, unlike the “prohibitory” financial punishment in Drexel Furniture. 259 U. S., at 37. Second, the individual mandate contains no scienter requirement. Third, the payment is collected solely by the IRS through the normal means of taxation—except that the Service is not allowed to use those means most suggestive of a punitive sanction, such as criminal prosecution. See §5000A(g)(2). The reasons the Court in Drexel Furniture held that what was called a “tax” there was a penalty support the conclusion that what is called a “penalty” heremay be viewed as a tax. . . .
Indeed, it is estimated that four million people each year will choose to pay the IRS rather than buy insurance. See Congressional Budget Office, supra, at 71. We would expect Congress to be troubled by that prospect if suchconduct were unlawful. That Congress apparently regards such extensive failure to comply with the mandate as tolerable suggests that Congress did not think it was creating four million outlaws. It suggests instead that theshared responsibility payment merely imposes a tax citizens may lawfully choose to pay in lieu of buying health insurance.
In other words, the individual mandate falls within the power of Congress because although it is called a “mandate,” it is actually pretty toothless. Despite the name, it’s not really a mandate.