The law itself gave little consideration to the prospect of states opting out, but most Republicans governors are rejecting the expansion as they try to undermine the Affordable Care Act however possible.
As a practical matter, Medicaid-eligible people were likely to fall under the law's other exemptions. But the administration clarified a direct exemption in Wednesday's rules implementing the mandate and its exceptions.
HHS also clarified an exemption for people whose incomes change over the course of a year.
The mandate doesn't apply to people who can't buy insurance for less than 9 percent of their annual salary. HHS clarified Wednesday that the mandate won't kick in retroactively for people who turn down coverage at the beginning of the year because it was deemed unaffordable, but then end the year having made enough money that coverage would have been affordable, after all.