Millions of people could qualify for federal subsidies that will pay the entire monthly cost of some health care plans being offered in the online marketplaces set up under President Obama’s health care law, a surprising figure that has not garnered much attention, in part because the zero-premium plans come with serious trade-offs. Supporters of the Affordable Care Act say that the availability of free-premium plans — as well as inexpensive policies that cover more — shows that it is achieving its goal of making health insurance widely available. A large number of those who qualify have incomes that fall...