A radical departure? Congress can tax inactivity

There is a mystery to John Roberts' opinion upholding the mandate under Congress's taxing power while ruling it an unconstitutional exercise of Congress's Commerce Clause power.  With regard to the Commerce Clause, Roberts adopted wholesale the plaintiffs' argument that it's a radical and dangerous departure for Congress to "create" commerce or "compel" inactivity.  Yet Roberts finds that Congress does have the power to tax inactivity, notwithstanding that none the precedents he cites as support for upholding the penalty for not buying insurance do tax inactivity. He is up-front about this apparent paradox:

There may, however, be a more fundamental objection to a tax on those who lack health insurance. Even if only a tax, the payment under §5000A(b) remains a burden that the Federal Government imposes for an omission, not an act. If it is troubling to interpret the Commerce Clause as authorizing Congress to regulate those who abstain from commerce, perhaps it should be similarly troubling to permit Congress to impose a tax for not doing something (p. 41).