Pornography and crime

According to the New York Times, 85% of those imprisoned for possession of child pornography also admitted molesting children. (H/T Brothers Bayly) Now, granted, there are some methodological caveats to this study, but one thing perplexes me.

How does anyone fail to realize that visual media are powerful in modifying behavior? When Madison Avenue makes billions by cleverly connecting a pretty girl with a product, how is it that people think that the portrayal of another pretty girl (or whatever) somehow does not affect behavior? I don't agree with Michael Medved on everything, but he nails this.