Right. The determinant as to whether information is classified is its effect on national security if widely known.
You don't classify innocuous material just because it might be embarrassing.
But on the other hand you can classify material that risks national security even if it also happens to be embarrassing.
In any event PRISM itself is 0.01% or so in actuality as it was claimed to be, I'm not even sure I'd call PRISM itself embarrassing (it's just a damn web service). The laws under which PRISM operate might be different, but that's not something we can pin on NSA, and it's not something that was secret anyways.
