Police officers in urban settings deal with vicious dogs all the time and clearly they've determined this Pit Bull isn't just wanting to be petted.
Now you may ask, "Why don't they just shoot it?" Well, one of the first things officers are trained on is firearms safety and one of the main rules is "Always be sure of your target and what's beyond it".
So they may have been trying to calm the dog down, hoping the owner would show up. Or the officer inside the car may be calling for Animal Control.
Hopefully this call ended peacefully with no one being bitten.
(Philly.com - June 3, 2013)