i happen to think they do know things that they can't tell while there is still a chance that Joran can be made to talk. that seems to make sense. were the case closed and then they were to disclose nothing as per their suggestion, i might agree with you. i think it is premature to reach the conclusion you have reached.
While I personally don't believe that Natalee is still alive, it can't be ruled out. Just the thought that she's out there waiting to be rescued would be reason enough to drive me until I found her. I think that that possibility, however remote, could serve to restrict what the Twittys say publicly. And it makes the Kaipoe's refusal to talk with them all the more reprehensible.