“The main argument for a mandate before the Supreme Court was that people of modest means can fail to buy insurance, and then rely on charity care in emergency rooms, shifting the cost to the rest of us. But the expenses of emergency room treatment for indigent uninsured people are not health-care’s central cost problem. Costs are rising because people who do have insurance, and their doctors, overuse health services and don’t shop on price, and because regulations have salted insurance with ever more coverage for them to overuse.”