When you engage on a forum, you invest something of yourself in it. It's like going to school. You dress up, comb your hair, sharpen your pencils and tie your shoes properly. You stand in front of the class to "Introduce Yourself!", then head on down to your favorite club... the "Opinions and Current Events" club, the "Movies" club or simply head down to class at the "Gran Turismo 6" room. Obviously, there are still a lot of spanners, but they get kicked out fairly quickly, and a core group of model and borderline acceptable students forms.
Comments on news articles, much like Youtube comments, are like bathroom walls. Nobody thinks they have to follow proper decorum. They're simply tagging them so other people will see their names. Granted, some of us comment seriously, but a lot of people don't give two :censored:s.