Your kinda missing my point...
If people dont want to exert the energy for mass graves...just stack them up on a bridge or across some county line to deter people from crossing into your territory. Two birds..one body.
As for the actual need for mass graves, I dont see much of a need normally. You wont find hundreds of people dead in the same spot often. Maybe a large group of refugees?
But they would most likely be strung out over a few miles...so unless its across your pasture, who cares?
In a more urban environment, if people start dieing off, most likely from lack of food, most people will move to another location rather then just sitting down and dieing. Again, another location means who cares?