yes, it's a mixed bag.
In some places the cameras are signaled up to 500m before, and are very visible with yellow reflective stripes. There is often one at the entrance of towns to force drivers to adapt their speeds, and it works.
Then some are clearly placed with malice, it really depends.
> Unless someone can prove a connection that someone poor is more likely to speed, then I don't see how that's true.
I think an issue would be that poor people will drive more. Middle class population will tend to be more urban and rely on public transportation for day to day use, even if they still own a car and may do a few trips during the week.
People in remote areas will be more at the extreme: crazy rich or pretty poor, and the poor in these area will have to drive everywhere to get almost anything, thus being more subject to speed traps. (anecdotaly almost everyone speeds way over the limit, some big highway feel like the autobahn sometimes)