It looks like they kept their silence so as not to look divided. My hunch is that the Kerry campaign and Viacom wanted to boost sales of Clarke's book, for differing reasons of course. The Clarke testimony was played up, leaked, whatever for these purposes, not for the work of the 9/11 commission.
In other words, as we always suspected, until Condi set things straight, the 9/11 commission was being used at Viacom and Kerry's will.
When the commission was interviewing Clarke, it seemed obious to me, that he was being used as a partisan tool for the Democrats. To Prove that they did this with that intent...amazing.