In order to make a point about a law that is unfair, it is sometimes necessary to break it. This is what activism is about. As long as the action is not harmful to others and is simply a demonstration of how the law is unfair, it is okay. A good example of this is the soda counter sit-ins during the Civil Rights movement. People literally just sat in white areas of soda fountains to demonstrate that it wasn't fair to have different colored sections. Their intent was not to harm anyone.
No, it is not ever justifiable for protesters and activists to break the law for the sake of their cause, because that is tantamount to terrorism. Destroying something of someone else's in order to make them agree with your point is how we define terrorism. Protesters would be more effective if they respected others' property.