Why did my dog just stand there while the postman was attacked by another dog?

I have a dark brown staffy male who is 3 years old. I was having a shower and turned it off and i heared screams of agony coming from the garden. I looked out the window and saw the postman laying in my garden being attacked by somones staffy. My dog was stood maybe 2 meters away and did not help, instead he just stood there and watched. I quickly got somthing on and ran down stares but i tripped and hit my head and i think i knocked myself out because i felt like i was there for ages. All i could hear was utter screams and cries of absolute agony. I got up and went to help. The dog had bit into the mans groin and would not let go and still my dog just stood there. I could not get the dog off at all so i ran into the house and into the back garden to get a shovel. I ran back into the garden and smacked it as hard as i could and it still did not get off. Other people got involved by trying to distract it and finally the dog stopped and police with these stick things came around the corner came in my garden and took it away. Everyone thought it was my dog till i said its not. I only have 1 dog and he is here. I heared that the postmans left testicles was biten in half . i dont understand why my dog just stood there. Why ?