It uses the commerce clause of the Constitution (Article 1, Section 8, Clause 3) – which governs activities – to make inactivity an offense.
… The individual mandate extends the commerce clause's power beyond economic activity, to economic inactivity. That is unprecedented. While Congress has used its taxing power to fund Social Security and Medicare, never before has it used its commerce power to mandate that an individual person engage in an economic transaction with a private company. Regulating the auto industry or paying "cash for clunkers" is one thing; making everyone buy a Chevy is quite another. Even during World War II, the federal government did not mandate that individual citizens purchase war bonds.
This was written by a law professor, at Georgetown University, and published in The Washington Post. None of those are regarded as conservative institutions.
Via Cafe Hayek.