Answer:
The United States did not need to spread propaganda, as they were not guilty of human rights violations. They were not eager to spread any specific political messages to their citizens. They also had a fair democratic system, and were a more civilized country. Additionally, the United States was not as interested in the prospect of a war with other nations.
Explanation:
Germany and Japan were not necessarily using the radio as propaganda, however, especially as early as the 1920's. This depends on one's perspective, as all world leaders were eager to communicate with their citizens through radio but were not necessarily spreading propaganda. Still, the German and even Japanese messages to their country were meant to spread ambitious political agendas that the United States did not have as much of a need for.
