For all the talk about freedom of speech and individual freedom in the United States, ours is actually a hierarchical society in which most people cannot afford to speak out unless they are themselves independently wealthy. A lot of Americans work for corporations, which would just fire anyone who became so outspoken in public as to begin to affect their company's image. Look at how many bloggers are anonymous! Purveyors of opinion in the mass media, who use their real names, are employed by, or in some way backed by, media moguls. It is fairly easy to depart from the spectrum of acceptable opinion (i.e. acceptable to the three million or so people who have disproprotionate weight in how America is run), and if one does, after a while one is not heard from so much any more. Thus, those attacking Kos work for Martin Peretz and Arthur Shulzberger, Jr., and if they didn't they would not have their current influential perches.
The very wealthy are used to getting their way in US politics and to dominating public discourse, since so much can be controlled at choke points. Journalists can just be fired, editors and other movers and shakers bought or intimidated. Look what happened to MSNBC reporter Ashleigh Banfield, who dared complain about the propaganda in the US new media around the Iraq War. Phil Donohue, who presided over MSNBC's most popular talk show, was apparently fired before the war because General Electric and Microsoft knew he would be critical of it, and did not want to take the heat. Politicians who step out of line can just be unseated by giving their opponents funding (the Supreme Court just made it harder to restrict this sort of thing).
There is an incredible amount of intimidation and intellectual thuggery that goes on in the internet -- and in day to day life. I, in effect, run my own business so I don't have to worry about being fired because I run this little repository or snark and rants. The same is true for the most part of my fellow blogboys, who have positions where they can affort to express their opinions. But we remain anonymous for good reasons too. Most particularly, I really don't care to have my other life interrupted by assholery of others.
It's what blogs allow.
Sadly, they also allow people with more money and bigger megaphones to margialize those who, by using psedonyms already marginalize ourselves.