Ian141, firstly welcome to the forum, and what a hot potato you've picked for your first post!
"Speed Kills" is just a slogan, nothing more nothing less. You are quite right in that as a statement it is incorrect, but as a slogan it works. "Inappropriate speed kills" would be more accurate but less catchy. I also agree with you that if the government seriously wanted them to be accepted as "Safety Cameras" they would be placed by schools and suchlike.
Poor driving is the main cause of accidents, I have no doubt whatsoever about that, however, it is convincing a person that their driving skills are lacking that is so difficult.!