Wouldn't that mean you have to build much, much more AAA to get the same effect? Instead of level 10 AAA in a line from Holland down to France, protecting all of the industrial cities, Berlin and whatever else is behind, now Schweinfurt, Berlin, Hamburg, etc. all need to have their own level 10 AAA. So instead of a bomber stream overflying X flak guns, they now only overfly Y, which is 10 or fewer.
A mission profile before would be Takeoff > Channel > Detected by radar > Coastal Flak > Main Defensive line > Target and returning through the same, taking losses all the while. I suppose lighter losses over the coast, many losses on the main defense, and losses over the target depending on the target's own defense. This means that over the course of the mission the bomber stream is losing aircraft almost the whole time, at variable amounts, and that any German city behind the line is still defended by those initial defenses.
Now, it would appear that only the province's own AAA is used. This represents fewer defenses total and also a shorter engagement time, since the aircraft are only engaged once they are already over the target. This also means that instead of say 200 provincial AA with 10 level 10 provinces as the flak belt (100 units), and the remaining 100 dispersed throughout, every potential target would need to be covered individually, meaning more guns, but less opportunity to fire.