I assume it's because the traffic laws were written so that it's illegal for a driver to do certain things. If so, owners of driverless cars could (at least theoretically) fight the tickets in court due to the lack of a driver to ticket?

If I were a judge I'd be tempted to consider the driver to be the person (or company) that caused such a car to drive on the public street, despite them not necessarily being inside the car at the time of the offense. After all, at some point in this process a person was involved even if it was someone at the manufacturer activating a "drive to the person who bought you" feature. (If it was an AI, then whoever created the AI and allowed it to do that, etc.)

But then again I have no legal training whatsoever, so perhaps that ruling would get me kicked off the bench or at least overruled on appeal :þ.