The post office collects an enormous amount of data about housing patterns and vacancies, some of which is provided to Housing and Urban Development Department and US Census to make policy decisions and provide data to the public.
Actually, the data in this article under-represents that number of people without indoor plumbing, as the map only shows data on the county level versus the census blockgroup level.
That said, the merits of indoor plumbing vs composting toilets that are properly maintained in rural areas. Certainly indoor plumbing wastes lots of water, that takes a lot of energy to pump around and then clean and dispose of.