The thing is that it is likely that a human driver would have seen the person walking and not paying attention to the road, anticipated a collision and sounded their horn. They may even have applied the brakes gently as a precaution and to alert following traffic. The person may not have stepped out, but the action by the driver would at least have no effect at all. Their action could have added more time to avoid an accident and hopefully brought the bus to the attention of the person. A robot is very likely to spot the human but would have trouble anticipating their actions or level of awareness. With this lack of empathy that a machine has, a 'safe' option would be for software could assume all humans were likely to jump out suddenly and keep slowing down and beeping, but this would be impractial. In the language of advanced driving; Instead of relying on forward observation and planning the robot driver is reacting to events, and that is more likely to lead to an incident.