The whole point of civil disobedience is that you are prosecuted to the fullest extent of the law.
When even the District Attorney (through his spokesman) announces that he won't prosecute those committing civil disobedience, he is encouraging lawless behavior.
In addition, they are not looking at the crimes, but at the "cause" when they decide whether or not to prosecute. In other words, if you commit crimes like trespass and disturbing the peace, while protesting the policies of the Bush Administration, you will not be punished. But if you engage in the exact same crimes while protesting abortion, you will have the full wrath of prosecutorial powers (RICO statutes, for example) bear down on you.