Non-bias jury usually means a change of venue, which was the case as soon as this case hit the media about a Mother lying about everything when her child is missing., which had already become a necessity before the first three days after Casey was arrested. Since change of venue was already a given, why not try to encourage Cindy/George/Lee to come to terms with Caylee being gone and let them try to get Casey to tell the truth, so they could bury the poor child?(Granted this did not work, but wasn't it worth a try?) We have no idea why they leaked that information, but they could have been trying to get someone we do not know to give up info they were holding because they could not believe that Caylee was gone.
As for a law to force the Police to release any and all information in an ongoing case, even before charges are brought, that is a horrible thought. If you would like to know why I think that, please go read this article.
http://en.wikipedia.org/wiki/Richard_Jewell