Few explanations were proposed to this fact, like the ones that while the violent episodes in the TV programs are accidental, and they are not targeted on encouraging antisocial behavior, the programs that encourage prosocial behavior are specially designed for this purpose. The programs that feature positive social interactions, altruism, helping etc, make people, and especially children who watch them, imitate the things they see.
The reason for such a great effect of the prosocial and antisocial modeling is that for many Americans the TV programs display their reference group, the people they want to associate themselves with. The people that say or do something on the TV screen are often seen as those who are worthy of imitation, as the information that is presented on the TV screen is exposed to much less amount of psychological and informational filters than the information got by the other means.
It's true that nowadays, when the TV screen substitutes parents for lots of U.S children, the impact of the TV on modeling their behaviors is great. The statistic says that an average U.S child spends 19-25 hours per week in front of the TV screen, thus it's not wonderful that the TV makes such a considerable impact on modeling of their behaviors.