By executive order last Thursday, New York Gov. Andrew P. Cuomo set into motion a state health care exchange, something mandated under the Obama administration’s Affordable Care Act. The exchange, and similar ones in a growing number of states, is intended to bring much-needed competition to the insurance market and help millions of uninsured Americans get coverage. Had New York failed to act, the federal government would have stepped in to impose its own version of an exchange, provided, of course, that the law survives the Supreme Court.