How did Chief Justice John Roberts interpret Congress’s commerce clause power in the Affordable Care Act cases?

In the 2012 Affordable Care Act cases, Chief Justice John Roberts rejected the government's argument that the individual mandate was a valid exercise of Congress’s power under the commerce clause.21

Roberts reasoned that the power to regulate commerce presupposes the existence of commercial activity to be regulated. He argued that the individual mandate did not regulate existing commercial activity but instead compelled individuals to become active in commerce by purchasing health insurance. Adopting a novel interpretation, he held that the commerce clause applies only to commercial "activity" and not to commercial "inactivity," concluding that the failure to purchase insurance does not constitute an activity in the ordinary sense.23

Although Roberts agreed with critics that the mandate exceeded authority under the commerce clause, he ultimately upheld the provision by interpreting it as a tax, which he determined was a legitimate exercise of Congress’s power under the taxing and spending clause.41