[I]t is estimated that four million people each year will choose to pay the IRS rather than buy insurance. We would expect Congress to be troubled by that prospect if such conduct were unlawful…. That Congress apparently regards such extensive failure to comply with the mandate as tolerable suggests that Congress did not think it was creating four million outlaws. It suggests instead that the shared responsibility payment merely imposes a tax citizens may lawfully choose to pay in lieu of buying health insurance…. Congress’s use of the Taxing Clause to encourage buying something is, by contrast, not new. Tax incentives already promote, for example, purchasing homes and professional educations…. The Affordable Care Act’s requirement that certain individuals pay a financial penalty for not obtaining health insurance may reasonably be characterized as a tax. Because the Constitution permits such a tax, it is not our role to forbid it, or to pass upon its wisdom or fairness.