There is a "vehicle" that each of us owns, however: our bodies. And while we don't "choose" to own them, we do choose to maintain them. If we don't insure, we self-insure -- that is, make what provision we can to pay for the health care that we will inevitably consume. But the self-insured rely on a massive reinsurer or stop-loss insurer whom they don't pay: the state. Federal and state law decree that no one may be denied care. Hence failure to carry health insurance is an economic decision that affects the larger community. "Self-insuring" constitutes economic activity that may be regulated by Congress.
That is the thrust of the 2-1 majority decision in the U.S. Court of Appeals, 6th Circuit, upholding the individual mandate in Thomas More Law Center v. Obama. Timothy Jost explains that Judge Boyce Martin, writing the majority opinion, redefined the question of whether the mandate regulates "inactivity" as opposed to economic activity:
Judge Martin contends that nearly every individual in the United States consumes health care, and thus must decide whether to purchase insurance or to self-insure. Self-insuring is no less an economic activity than purchasing insurance. Self-insuring results, however, in a substantial cost-shift when those who choose to self-insure cannot in fact cover the cost of their care. For 2008, $43 billion dollars worth of care was passed on by those who chose to self-insure to others. Thus self-insuring has a substantial impact on interstate commerce.In fact Judge Martin also asserted that there is nothing in the Constitution or case law to prevent Congress from regulating 'inactivity" if the so-called "inactivity" affects interstate commerce: "the text of the Commerce Clause does not acknowledge a constitutional distinction between activity and inactivity, and neither does the Supreme Court" (p. 23).
That said, Martin also argued positively that the individual mandate "regulates active participation in the health care market" (p. 26, my emphasis). Here again, the concept that those who do not buy health insurance effective self-insure is key: no one, insured or not, is inactive in the health care market; insurance determines who pays; and those who self-insure in aggregate pass their costs on to the insured. Those who self-insure do so "cognizant of the backstop of free services required by law" (p. 19).