This is Jonathan Gruber. He's the MIT guy that is the brains behind Romneycare in MA and Obama used him to craft Obamacare. He admits that the law is set up so only people that get signed up on state exchanges can get the tax credits. Watch around 31:30.
Here's the quote:
What’s important to remember politically about this is if you're a state and you don’t set up an exchange, that means your citizens don't get their tax credits—but your citizens still pay the taxes that support this bill. So you’re essentially saying [to] your citizens you’re going to pay all the taxes to help all the other states in the country. I hope that that's a blatant enough political reality that states will get their act together and realize there are billions of dollars at stake here in setting up these exchanges. But, you know, once again the politics can get ugly around this. [emphasis added]