Speed cameras are usually set at 2 places : 1. Accident blacksopts 2. Revenue Generating places.
Turns, corners, intersections, junctions fall under accident blackspots - the point you've mentioned is also an accident blackspot.
Secondly - it is a common misconception that speed cameras have to be 'warned' about. The speed limit is a traffic warning. If one can't comprehend that, he shouldn't be driving.