In the case of the violent crime they believe it is because the people that commit crimes (young males) also like violent movies. When they are at the movies drinking their sodas they aren't on the streets drinking their beer and looking for trouble.

In the case of rape and Internet access it's more of catharsis. Let just say they can use the easily available porn on the internet in the privacy of their own room in ways they couldn't in a laboratory setting. And it's easier to close the browser window than it is to hide the porn magazines from the parents. Not to mention getting hard copy porn is more difficult than internet porn. At least this is the working hypothesis of the researchers.