In the beginning radio was completely run by amateurs and that was fine with the government. This lead to increased development of technology. These amateurs later became those who were pioneers in the commercialization of the national radio industry. After WWI people began to fear the culture of the nation was splintering apart due to immigration and the local broadcasts that were catered to certain segments of the population. The Government chose to step in and create a national radio system first as a way of uniting the nation.
The government first forced a British company to sell the rights of the airways, so the U.S. government could choose who could have permits. Still at this time permits were being given out quite freely. RCA then created guidelines saying that ownership of all stations must be at least 80% Americans. I believe the stress for a national broadcasting system was first done for cultural reasons, not economic ones.
When NBC and CBS entered the market people we worried of a monopoly. They still desired the local programing that was tailored to them, and did not want propaganda. NBC and CBS were able to use thier financial power to force amateurs off the air. This is because of the redone Radio Act, in which people or companies could buy certain radio frequencies. With their deep pockets the national systems won over Americans with their much more entertaining radio programing. Local radio was still provided to people but only when national companies allowed them to air. Shows became sponsored by companies, so while they did not have the advertising breaks like todays radio, a specific product would be mentioned multiple times in a radio spot.
NBC had a specific set of channels for educational programing that they put on less used frequencies. The Government knew that armatures could no longer compete with the large corporations so they mandated that some airtime must be set a side for “public service use”. National broadcasts met this mandate by getting their educational programing sponsored by companies. Radio was changed to preserve our culture. In a way you could say “if people can find a way to make money off of something they will” is actually in our culture.