We do this because speed limits are arbitrary. The safe speed on any patch of road varies by: vehicle, weather, traffic, driver ability and alertness, lighting, etc. That's why police and courts have discretion. Sometimes 20 over is safe, sometimes 20 under is dangerous.
There is a difference between “20 over is safe” and people often get away with doing 20 over. It’s safe, until it’s not, and then it contributes to a crash happening and/or being more severe. It’s safe until shit happens. And we know with humans, shit inevitably happens.
I think the speed limit should be the limit/maximum and then people can slow down further if conditions require
At least in Ontario, speed cameras are only allowed in community safety zones so the environment dictates low speed because of the presence of children, seniors, and families in a complicated road environment full of transit vehicles, bicycles, pedestrians, trees, playing, and other vehicles.
The asymmetry of gains and losses matter here. The gains are minuscule--a few drivers might get to go slightly faster? And the costs are measured in blood.