Outposts are run by noble families, placed at strategic locations. The city is run by the main house, and the head of that family. So outposts are simply a noble family legitimately attempting to secure supplies for the city. of course these can then become cities unto themselves.
If your outpost becomes a full fledged city because you have the supplies to maintain that many people then your family is obviously the main house and thus you are considered the lord of that city.