Workinghard,
You need to balance the benefit obtained from training with the risks involved:
Flying low over rural areas = high training benefit, moderate risk to aircrew, minimal risk to civilians on the ground (in an ejection)
Flying low over built-up areas = high training benefit, moderate risk to aircrew, substantial risk to civilians on ground (in an ejection), excessive disruption cause by noise, and a quick way to pi55-off joe public!
Basically, it ain't worth it!
Kind of obvious, I thought.