State officials in Virginia refused to create a State Exchange so they wouldn’t be on the hook for service provision. Instead, the Feds bore the burden and created an Exchange with the understanding that this included the IRS applying a tax credit to Virginians. This tax credit forced the income threshold down and ultimately expanded coverage to thousands more Virginians.
So because the credit increases the income of the petitioners, they’re now forced to buy health insurance or be penalized by the IRS. That’s why they went to the Supreme Court who basically said that Virginia should have known before agreeing to the Feds creating the Exchange.
Virginia didn't agree to the federal exchange being created. HHS created it when Virginia said it would not create its own. No State could prevent HHS from creating a federal exchange in a State that did not create its own exchange.