“Fairly stated, this is the conservative constitutional argument: Health care for all is a good cause. But if, in the name of that noble goal, you construe Congress’s power to regulate interstate commerce so broadly as to encompass individual choices that have never previously been thought of as commercial, much less interstate, there would be nothing left of the commerce clause’s restraints on Congress’s power. And then, the argument goes, Congress would be free to impose far more intrusive mandates.”