Thanks for all the thoughtful feedback.
Just a few notes: Unfollow doesn't work for search results, mentions, or for any other selection of tweets that is not strictly based upon your followings. Even with your followings, for example, a news organization may wish to occasionally signal graphic content to its followers, but not wish to lose you as a follower. Potentially, this allows the writer to balance journalistic expression in a manner consistent with an account with broad reach.
Twitter has consistently and publicly taken a strong stand on freedom of expression. For example, see this recent blog post, "The Tweets Must Flow". http://blog.twitter.com/2011/01/tweets-must-flow.html