The thing is "no comment" would basically confirm that 'something' is happening that's secret. Everyone would go conspiracy wild and pull everything they have away from Google.
They probably want to confirm it, but in a completely open and transparent way that assures people there's nothing they should fear here, which they can't do because it's all cloaked in secrecy.
So the answer they give when asked, and they do get asked, is "no comment."
If they were voluntarily part of PRISM, and legally required to keep that quiet, I'd expect them to say "no comment."
Doing the opposite make so little sense. It means they're having to flat-out lie to their users, something very hard to recover from.