Chief Justice Roberts was entirely correct in holding that the Affordable Care Act's individual mandate could be sustained as a tax. But in describing why that was the case, Roberts also revealed a fatal flaw in the section of his opinion that dealt with the Commerce Clause.

Roberts began his Commerce Clause analysis by stating: “Congress has never attempted to rely on that power to compel individuals not engaged in commerce to purchase an unwanted product.” This clever phrasing allowed him to exclude from his claim the gun mandate (as not under “that power”) and the prior mandates requiring shipowners and seamen to buy medical insurance (because they were “engaged in commerce”). Roberts then ruled that the Obamacare mandate went beyond the Commerce Clause, but nonetheless sustained it as within the taxing power.

But Roberts' tax argument actually undermines his argument about the inapplicability of the Commerce Clause. Roberts reasoned that Obamacare really imposes a mandate only on those subject to its tax penalty—which is limited to those who have thousands (probably tens of thousands) of dollars in earned income. What Roberts seems to have missed is that you cannot have earned income without engaging in commerce. (Gift income does not count as earned income subject to this tax).

Thus, Robert’s own tax reasoning means that the Obamacare mandate applies only to those who are engaged in significant commerce; in other words, the health care mandate was never being applied to people who were “not engaged in commerce,” the criterion so central to his Commerce Clause analysis. Since the insurance mandate regulates only those who are indeed engaged in significant commerce, it ought to fit easily within Roberts' own reading of the Commerce Clause.

But later in his opinion, Roberts does something sneaky. Although his textual reading of the Commerce Clause and his claim about lack of precedent both turned on the claim that the mandate applied to someone engaged in no commerce at all, he later writes that the main problem was that “most of those regulated by the individual mandate are not currently engaged in any commercial activity involving health care.”