The answer is that in passing the act, Congress was relying partly on its authority to regulate interstate commerce.

ECONOMIST: Rape and states�� rights

Upholding the Affordable Care Act under the Commerce Clause would give Congress the same license to regulate what people do not do.

FORBES: When Is A Penalty A Tax? Sorting Through The SCOTUS Health Care Decision.

Justice Ruth Bader Ginsburg wrote from the liberal side of the bench, urging the court to uphold the Act in its entirety and rejecting the Roberts view that the individual mandate exceeds the power of Congress to regulate interstate commerce.

FORBES: Obamacare Dissents Poke Holes In Roberts' Reasoning