TV itself does not have a negative effect on morals. People are not likely to watch a show then suddenly decide they believe differently than they did beforehand and throw their morals to the wind. Anyone who does react in that way most likely had many other factors at play and doesn't show for the general population.
Morals are something learned throughout childhood and adolescence, and are generally picked up from family life and socialization. Children learn from their parents, teachers, friends and the community as a whole, but TV is just a method of entertainment. While there are certainly some shows that address moral issues, the vast majority of what is on TV has little to do with morality at all, and does not affect moral development.