The marketplace of idea belongs to a libertarian political system, which is usually embodied in a capitalistic economic system. It would most likely be found in some sort of democracy, or a pluralistic political system that values the ideas of everyone more or less equally. The idea behind the marketplace is to allow and even encourage a diversity of opinions, with the understanding that people are ultimately rational and will eventually reject falsehoods and make wise choices regarding their lives. The media would be free to print or broadcast whatever it wishes. The media's job to provide the public with what it wants.
Libertarian thought believes in the rationality of humankind.
People may be exposed to untruths, but they are ultimately wise enough to sort through the arguments and make the correct decision. Social Responsibility partially agrees and partially disagrees. It agrees that people have the potential for rational thought. However, it thinks many people do not exercise this potential. So, they must be protected from themselves, in a sense.