Cameras are soulless and brainless. They cannot differentiate between good and bad, they can only work within set parameters, and the parameters can never be wide enough to take in human behaviour.
A policeman at the scene of a speeding incident has the ability to differentiate between good driving and bad, and can react accordingly. A motorist driving at 20mph over the limit on an empty road in good driving conditions is completely different to some drunk meandering between lanes in the pouring rain at the same speed on a busy road.
But they'll both be treated the same if caught by camera.