Isn't it possible that the reason few people criticize Alyssa Milano (or anyone else) on Twitter is the fact that you are, in essence, criticizing someone to her face? The searches you bring up all seem to include the "@" prefix, which is a means of directing commentary and traffic at a particular person, and in response drawing their attention toward you. I doubt many of the anonymous internet people who write nasty things about celebrities would say these same things while sitting at a table next tot hose celebrities. And Twitter functions the same way. The fact that the person about whom you are speaking is able to hear what you say greatly changes the dynamic. Therefore, many critical posts probably don't include the "@" and therefore don't show up as easily in your search.

This is true even of politicians. A search for @chuckgrassley certainly isn't all roses and sunshine -- he's the subject of several re-tweeting campaigns on the public option and net neturality -- but none seems especially vicious. But a search for his name without @ retrieves both "Chuck Grassley is a WHORE" and "I have seen Satan and his name is Chuck E. Cheese/or at least Chuck Grassley."