Because there’s still someone in the car, they just have no way to defend themselves. You can tailgate and honk at them to your heart’s content. Well at least until they call the police but that’s pretty far. And there are other forms of aggression that do accomplish something. If you cut a Waymo off or beat it to merge, you get ahead of it. In some locales I could see a whole series of cars merging ahead of a Waymo if people are aggressive enough.