The overriding social policy objective isn't to effect urban-to-rural subsidies per se, but to provide certain types of basic infrastructure universally — and it just so happens that the unit economics point toward a wealth from urban to rural areas as a means of accomplishing that.
To the best of my understanding, the consensus around this formed out of the Depression and was bolstered subsequently by post-war thinking. The idea was that we as a society should be able to make certain social guarantees of living standard and access to services.
Lots of things gave rise to that strain of thinking across the Western world, some of which were in response to specifically American conditions, others not. To the extent that it was influenced by the same kinds of currents that gave rise to the Western European welfare states, there was a pattern of concern over the ways in which savage inequalities between country and society perpetuated intergenerational poverty, which was in turn exploitable by demagogues and populists. There was a time in American history when the society was moving in the same general political direction as Europe as far as universal social benefits, and while the trend was unquestionably weaker here, it give rise to Medicare and Medicaid and other LBJ War on Poverty-era initiatives. Another, less acknowledged motivation was to defeat the allure of the Soviet model; revolutionary Bolshevism was big on redressing the historical gap between the sophistication and modernity of the city vs. the backward depression of the country, and universal literacy, universal electrification, universal medical infrastructure, etc. were widely touted goals. And of course, this factor was even more pronounced among the European social democracies.
The more locally American consideration was most likely that telephone service should be folded, together with farm price supports, into the overall package of stabilising subsidies to the critical agricultural sector. This was thought necessary to cushion them against a repeat of the acute economic devastation of the Depression, the Dust Bowl, etc. Telephone access was essential to economic development and automation in the twentieth century in the same way that Internet access is regarded to be now, and was practically important for farmers as a means of getting their products to market.
Last but not least, participation in the social bargain of universal service was generally a condition of being granted some sort of natural monopoly status, as for example with electrical utilities. The same general thinking was extended to the telephone, since AT&T did, after all, monopolise the local loop in most of the country for much of the twentieth century.