These politicians are politicians, not scientists. They use their romantic ideology to try and explain, and fix, the world. When a Senator says violent video games are to blame it is because of their own personal beliefs. For the same reason many people like bonkers555 agree with them. This is the same thing as people disliking "Frankenfood" or immunizations without any scientific reasoning.
Violence in video games is not to blame for mentally disturbed people committing horrendous crimes. Video games and hobbies in general that isolate individuals from human interaction can lead to the devaluing of human life but that's not enough to get a person to pull the trigger.