NorthBay Adventure is the kind of small business that could be expected to buy medical insurance for workers under sweeping health-act rules taking effect in 2014. But executive director George Comfort says that’s not likely to happen. Instead, NorthBay became self-insured last year, paying most of its workers’ health costs directly, a practice more typical of large employers. The decision to self-insure was about free choice, savings and what’s best for his company, Comfort says. But others see it as a threat to the Affordable Care Act. As more small employers like NorthBay avoid the health act’s requirements through self-coverage,...