User blog comment:B214/General Discussion/@comment-32559006-20170726163611/@comment-10330822-20170730221216

Gremory and Sitri have control over certain facilities in the city. For example, Zeoticus is an administrator for Kuoh Academy. My guess is that they bought a lot of the city when Rias decided she wanted to study there, so they could accomidate her and her devil work as much as possible.

Also, the territory doesn't have to be literal, more like a supernatural property statement for who gets to do business where. The Gremory Clan does business is Issei's town, and they alone are allowed to do so. Sona, who goes to the same school, probably also has territory in the town that her group does.