It is one thing to request businesses for compliance and assistance to carry out the law (like a policeman coming into a business asking for assistance when a delinquent did business there) and another is for the business to be responsible for the delinquent's actions. Websites should not be made to answer for a poster's lack of values! That is not how real life works - it lacks common sense!
Internet will likely at some point follow real life (since it's proven you really can't be anonymous in the internet). Let's remember the internet is a big, distributed expression of human behavior; oppose anything that does not make sense!