Mercinae Justice enforced east of path entrance as mercinae land a long time ago. Over the last few months, the boundaries appear to have shrunk purely to the city walls, rather than city owned land. Rather than say - you can't attack at the shop, or the barraks, but you can attack on the two path locations between them, it is a lot easier just to protect south of the shop and east of p. e. to ensure the safety of my younger citizens trying to do their gold runs.
The area covered by Justice is purely related to it's ability to enforce it. And though I might not be able to kill the likes of Astiron and saxe without thinking, there are an awful lot of people I can kill with little effort (which to be honest, is rather a poor reflection on modern thakrians).
If you want to query the safety of my citizens derillion, I can always give you a personal demonstration.