From the dissent:

Words no longer have meaning if an Exchange that is not established by a State is “established by the State.” It is hard to come up with a clearer way to limit tax credits to state Exchanges than to use the words “established by the State.” And it is hard to come up with a reason to include the words “by the State” other than the purpose of limiting credits to state Exchanges…Under all the usual rules of interpretation, in short, the Government should lose this case.