And that can (and does) happen with just about any dog breed. It happens more often with pit bulls because that is the trashy losers' dog of choice.
You're right, any breed of dog is capable of an attack. However, the fact that 67% of fatal attacks are committed by a breed that makes up 5% of the dog population is a fairly telling statistic.
In reading up a little further on this, I came across a few sources that agree on a basic difference between dog attacks by breed. According to many of them, pit bulls (and to a lesser extent, rottweilers) attack to kill, while almost any other breed (including German Shepherds) attack to intimidate.
Responsible ownership does lower the risk, but to suggest that any inclination toward aggression is purely environmental is a hollow argument.