I agree with you, they are indeed rewriting the already existing text on the internet. We consider such duplicate content issues as a part of SEO, but in this case we need to make sure that not all websites are SEO driven – most of the twitter bases services are driven by tweets, tweets and tweets only. More the number of followers, more the visitors they get – they don’t need to care what Google thinks of them.

Just for example I am pasting the screenshot of google results for the top item at retweet.com, you can observe the original blog post is ranking at top followed by other blogs, twitter’s original tweet > a retweet and nowhere retweet.com or tweetmeme.com hold a place.

You can read an interesting interview with tweetmeme here: http://technotip.org/interview-with-tweetmeme/