> Graffiti is a population's expression of ownership of their city.

I think this is the heart of it, and where cities and suburban towns differ.

It's admittedly very hard to articulate in words. The walls of buildings in a city are part of the greater, broader, "face of the city." They are in a sense both part of a general "public space" yet also still privately owned. The walls of single family homes in suburban neighborhoods don't really compare. There's much more of a shared sense of "ours" in a city than there is out in the country, where everything's fenced off in little discrete boxes of land, each with someone's name on it. This greater sense of shared agency over the aesthetic of the broader "city" makes street art more justifiable there than it is in single family home places.