I think this action, and several other "leaks" which have occurred, are there to help prevent an "outrage" over whatever the grand jury decides. By doing things like this, it is a suggestion there will not be an indictment against the officer. Preparing people for an outcome will always elicit a less emotional response than catching them by surprise.
I think Governor Nixon is doing the best he can to make sure the peace will be kept. To anyone who disagrees, I seriously wonder what you think should be done differently.