When a cocker spaniel bites, it does so as a member of its species; it is never anything but a dog. When a pit bull bites, it does so as a member of its breed. A pit bull is never anything but a pit bull.
My dog Harrison (below) is about as sweet as a dog can be, but he always gets attacked by other dogs and they've been either pit bulls or chows that attack him. I love all dogs, all breeds etc. and I realize that any breed of dog could potentially go after him but at the dog park we go to, it's twice been a pit-bull and once been a chow that have gone for him, unprovoked. -That's my personal experience.
That said, a number of smaller dogs have been aggressive towards him, but they don't have jaws like vice grips.
Still, I agree that when a pit bull does something "bad" it's thought to be because its a pit bull but any other dog, it's just a dog being aggressive and isn't necessarily attached to the specific breed. -It's a double standard.