Not sure what the philosophical argument is, but it sounds like we're using artificially inflated prices (due to lack of competition) in urban areas as a sort of roundabout way of providing mail to rural areas. The comment to which you replied seems to suggest that it's better/simpler to just provide loss-making mail services to rural areas funded through progressive taxes and be done with it, like we do for other services we deem socially necessary (military, education).