Roberts swallowed and regurgitated all of that illogic because, unlike the four dissenting justices, he was not prepared to overturn the health care law but, like them, could not accept the notion that forcing people to buy stuff counts as regulating interstate commerce. He warned that "allowing Congress to justify federal regulation by pointing to the effect of inaction on commerce would bring countless decisions an individual could potentially make within the scope of federal regulation and -- under the government's theory -- empower Congress to make those decisions for him."
But recasting regulation as taxation only magnifies this danger. Under the government's view of the Commerce Clause, Roberts worried, Congress could order Americans to buy vegetables, citing the impact that poor diet has on health care costs. Under Roberts' view of the tax power, Congress could do much the same thing, imposing a levy on people who fail to buy vegetables. It would not even have to allege a "substantial effect" on interstate commerce. Indeed, Roberts said "the breadth of Congress' power to tax is greater than its power to regulate commerce."
By way of reassurance, Roberts added that the former approach "leaves an individual with a lawful choice to do or not do a certain act, so long as he is willing to pay a tax levied on that choice." People who decide not to pay a tax, of course, can get into plenty of legal trouble, including criminal prosecution as well as liens and forfeiture.
For political reasons, Congress barred the IRS from using those scary remedies to extract the "shared responsibility payment." The insurance mandate was controversial enough without raising the prospect of taking people's homes or throwing them in prison if they failed to obey it. Future "tax" legislation aimed at getting Americans to behave as members of Congress think they should may not be so gentle.
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