What real difference is there between an urban area and a rural area that would make such a great difference to low level fast jet operations? You have tall buildings in urban areas - you have tall trees, hills and buildings in rural areas. The aim is to fly above / around them and not hit them - same rules would apply whether urban or rural. Topography is topography, whether covered by crops or buildings. So why risk complaints of thousands (or worst case the lives of thousands) while training where you will get the same training value over less habitatd areas?
Similarly, surely your avetrage pilot would want to avoid population centres (unless of course they are the target), therefore avoid being seen and reported on / shot down?
Its already been mentioned that helo operations already train in the uban environment - the ones most likely to need to use that skill.