Noone cares? Really? If you were a public person, such as a Lord and suddenly there was this massive retweeting going on about a tweet, that states that you are a child molester (in a serious way, not a joke), you wouldn't care? Really?
On second thought, going after the retweeters is pretty dumb, though i guess it depends on who retweets it and in what way. In the same newspaper analogy, if the newspaper simply printed "Person A is a child molester", with person B as source on the bottom, it could very well be held accountable for libel, afaik.