Interesting point. It would be nice to think that they picked it up to put it to bed early. Given that Scalia was so upset about it, I would think that he didn't vote to pick it up. Then again, I can't see the progressives picking it up unless they were quite sure that it was going to be upheld. Perhaps Scalia was so pissed at Roberts because he felt betrayed. My guess would be that Scalia, Roberts, Alito and, Thomas voted to pick it up.
Deciding not to hear a case is a de facto way of ruling, since it basically means, "we agree with the lower court." The logical extension is that we should know who is ruling what way. They're obscuring an essential function of government as top secret (but not really, because even top secret stuff often becomes public after a number of years). It's a disgrace, IMO. What if members of congress were allowed to operate in secret? We'd never stand for it.