Lang Zhen's words were not a threat. There were truly too many examples of runners who had committed wrongful acts being killed outright by others.
What does "imperial authority does not extend past the county" mean? This is precisely it. In the regions below the county level, aristocratic families hold significant sway over discourse.
Many clans could directly decide the life and death of their own members, and even that of their neighbors, forcibly if necessary.
If runners were dispatched and the locals had no choice but to flatter them, they would generally do so. However, if the runners overstepped, the locals might kill them, hide the bodies, and still might not necessarily face any consequences.
If the officials must pursue the matter, what if it provokes a backlash from the local populace? Whose fault would that be?