Traffic cameras catch a much higher percentage of violations than the old fashioned “cop behind a billboard” method. The size of fines evolved to create sufficient deterrence, given the low probability of getting caught. IOW, we (the potential offenders) would consider both the size of the fine, and the probability of getting caught, when deciding whether it’s worth while speeding, running a yellow light, or whatever. Here’s a simple formula for the deterrence effect: Low probability of being caught X high fine = high probability of being caught X lower fine.
With traffic cams, the probability of getting caught is much greater. That means that a smaller fine should create sufficient deterrence effect (especially when combined with a points system, that would result in a license suspension for multiple offenders.)
Naturally, it has never occurred to any government to lower the fines, when traffic cams are introduced. Hence the oft-heard accusation that the cameras are all part of a cash grab.