One of my uni friends witnessed one of them happening on the underground. Some school kids were just staring at some bloke falling asleep , slapped him across the face as they ran out the door. He said the bloke was more in shock to what had happened than wanting to fight back.
I think its definitely something to do with he fact that people just cant understand whats going on. If someone actually wants a fight with you then atleast you know it and can get sort of prepared for it, but with the random slapping and shit its just out of the blue so you get the major shock and instinct is just to get away from the pain which is why most people just walk away... quickly.
Also no one else usually gets involved since nowadays you never know what people are going to be carrying (knives etc.), its always the do gooder that ends up in hospital bleeding all over the place.