I'm curious about this too. It's highly irregular to announce bail before a suspect is even brought in and arraigned.
Here's the only logical explanation I can come up with:
The authorities are afraid that MJ will flee the country outside of U.S. juridiction. He certainly has the resources to disappear like that and still ensure a very comfortable life somewhere else with plenty of flunkies to protect him and cater to his ever need (such as supplying young boys).
So in order to prevent him from fleeing like this, they announce to the media about the $3 million bail. This has the MJ camp probably thinking "Hmmm, we can make this bail. Let's show up for the arraignment and see what they (the prosecution) has. Then we can make bail and decide what to do later."
So basically, the intent is to not have MJ worry about spending a night in jail. He'll surrender himself, thinking that he'll be back in Never Neverland in a few short hours where he can then have plenty of time to prepare his exile before the trial even starts.
But maybe the authorities have a little surprise for MJ. Once he surrenders and is arraigned, he is hit with so many charges that the judge decides to hold him without bail until trial and classifies him as a danger to society.
One can only hope.