When I first learned of the conflicting ruling between the DC Appellate Court and the Fourth District Court of Appeals, I went back and research the PPACA reviewing those provisions of the law that applied to the States creating their health insurance exchanges (Sections 1311-1313) and the tax credits that would apply (Sections 1401, 1411, 1413 and 1415). When I read these sections, I interpreted the meaning of the word "State(s)" to mean the same as it would mean in the 10 Amendment to the Constitution - all 50 State - which is the same way I'm sure pretty much everyone else would interpret "the State(s)" would mean in this case. Cut and dry, right? Not quite. You have to read Sections 1311 and 1321(c)91)(B) very carefully. The Secretary of HHS does have lots of leeway in setting up Exchanges in those states who don't establish them AND in issuing tax credits for those who qualify. (Mostly those who purchase Silver plans, Section 1331(a)(2)(A)) There's also the Healthcare Reconciliation Act that according to the below linked article attempts to clarify any vagueness on this matter. I admit I haven't read the provision(s) that apply from the HCRA because last time I read it, it mostly dealt with funding education initiatives in the medical fields (i.e., doctors, nurse practitioners and nurses).
I found this article that does a very good job of explaining the situation (or should I say "did" considering that article was first written in 2012 in anticipation of a lawsuit on this very tax credit issue).
Tax Credits In Federally Facilitated Exchanges Are Consistent With The Affordable Care Act’s Language And History – Health Affairs Blog
Regardless, this issue may well need to be settled by the SCOUS.