Example 1: Red light cameras. Only needed at specific intersections that have particular characteristics (high speed? high volume? not sure). The distribution of these probably maps more to physical space than population itself as the spacing of red lights is relatively independent of population.
Example 2: Cameras that are aimed at crime suppression. These will be unevenly distributed around a city depending on where crime happens. Looking at Chicago I see particular neighborhoods heavily blanketed in cameras, others very very sparsely. The distribution of this type of camera would be quite different than traffic-control cameras.
Example 3: Population monitoring cameras. I honestly don't know why China has so many cameras. Are these literally population surveillance cameras? These might be evenly distributed around a city to maximize coverage.
Some of the power law distribution you see might be a result of different cities applying cameras for 1, 2, 3, N different functions (each of which requires a different number of cameras).