In the New York Daily News, law professor Ilya Somin writes:
The health insurance mandate imposes a fine as punishment for the unlawful refusal to purchase government-mandated health insurance. The chief justice writes that this is not a penalty because “the mandate is not a legal command to buy insurance,” but merely a requirement that you pay additional money to the IRS if you refuse to comply. Failure to purchase health insurance, is therefore, not really “unlawful.” This distinction is hardly persuasive. Is speeding not really unlawful if the penalty for it is a fine payable through the IRS?
Pretty much any other mandate could be magically converted into a tax by the same sleight of hand – so long as the penalty for violating it is a fine similar to the one that enforces the individual mandate. The danger here is not just theoretical. Numerous interest groups could potentially lobby Congress to enact a law requiring people to buy their products, just as the health insurance industry did.