“Free” preventive health care is one of Obamacare’s chief selling points. But as millions of newly covered people begin to seek that benefit, some are still getting stuck with bills. The Affordable Care Act requires insurers to pay the full cost of services like cholesterol checks, women’s birth control, immunizations, colonoscopy screenings and a host of other items. It also covers “well visits” for children and adults — periodic checkups not triggered by a particular health complaint. But does a free colonoscopy cover just the screening or the immediate removal of polyps, too? Does the smoking-cessation benefit include counseling, medication...