In striking down the individual mandate in the new health reform Act (and then the rest of the Act too just for good measure), Judge Vinson parroted the common argument that if Congress "has the power to compel an otherwise passive individual into a commercial transaction with a third party merely by asserting --- as was done in the Act --- that compelling the actual transaction is itself 'commercial and economic in nature, and substantially affects interstate commerce' [see Act § 1501(a)(1)], it is not hyperbolizing to suggest that Congress could do almost anything it wanted." As I've argued before, though, this is just plain wrong. The Supreme Court's existing Commerce Clause doctrine, under which it has struck down laws that bore no genuine connection to economic matters or encroached on areas traditionally left to State regulation, applies to legal mandates as much as to legal prohibitions. Some mandates (including the one in the health care Act) pass the test; others wouldn't.
Of course, some folks think that existing limits on federal power are too lax (I don't), but that has nothing to do with the activity/inactivity distinction. And if those critics wanted to tighten limits on federal power in ways that actually responded to some coherent and sensible ideas about federalism, the last thing they would do is just pull an activity/inactivity distinction out of thin air. They might, for example, want to demand that the intrastate behavior that a law addresses actually have something to do with a national economic market whose effective regulation arguably requires a coordinated national response. Some federal laws would fail such a test. But an individual mandate to buy health insurance as part of a comprehensive rethinking of the national health care system would pass it easily.