It's a flawed rule in built up areas, and in situations like minor roads without pavement near a village, where you'd expect to see pedestrians, dog walkers, hikers etc. Many of those spots would benefit from a camera on nearing village outskirts and 30mph limit. Yet they normally don't have one.

On dual carriageways, which is about the only location I ever feel annoyed by speed cameras, it's a fairly reasonable basis for setting limits. That's the situation you find reductions of limit, with associated speed camera, that are simply unexplainable from road conditions, and lack of sources of danger, risks such as built up areas etc.

I'm pretty confident horses, especially ones pulling carriages, aren't super safe for pedestrians either.