Wanna know my theory? I think Locke told them to keep their mouths shut about it. We know from the little snipet from the preview for this weeks episode that Locke talks to Jack and that what's he going to do is going to be awesome. So my guess is he tells Jack to not say anything about the Island so as to keep it safe from anymore outsiders. Obviously, as we've seen from the Freighter folk, outsiders are not good for the Island.
However this also leads me to the unfortunate conclusion that this also means our beloved Locke is going to die. Who else would Kate say "No, why would I?" when Jack asks her if she went to the funeral? Perhaps Locke dies saving the Oceanic Six, and successfully moving the Island, but giving his life in the process. That would suck to no end, but also seems to be the most likely course of events.