Because they should stop like ordinary traffic. It could cause an accident and if the person on the bicycle get's hit, it usually always gets blamed on the driver of the car.
Why do police target cyclists who jump red lights?
A bicycle is considered a vehicle in all municipalities I have ever lived in and vehicles are to stop at red lights.
No one targets bicyclists, as they usually get away with this, but it is dangerous behavior for obvious reasons. A man in San Francisco went to prison not too long ago for striking a pedestrian in a crosswalk and killing him. He ran a red light also.
Perhaps if the police really targeted such people ( many other accidents happen short of death ) then this and other accidents would not happen.
A bike is supposed to follow the rules of the road as a car should. If you run a red light you are breaking the law, the same as if you were in a car.