I thought yesterday about the circles also and the only thing what came to my mind which makes sense is the following.
A speedcamera should only be in a certain range, for example, if driving on the street it makes only sense that the speedcamera covers the current driving direction and range. If a circle is defined, the opposite traffic would not be affected (no alarm) cause the speedcamera is, when driving on the opposite lane, more then 50m or 80m away.
Hope I could explain it understandable