If Twitter receives a complaint that a tweet has breached copyright, the site will now transparently display a notice explaining why the tweet was pulled instead of just yanking the infringing tweet.
A policy shift will now see infringing tweets replaced with a warning, such as: "This Tweet from [username] has been withheld in response to a report from the copyright holder," along with alink to Twitter's copyright policy and the Digital Millennium Copyright Act. Other Twitter users will also be made aware that the tweet was withdrawn and can reply to the withdrawn tweet notice asking why it was taken down.
Source: CNET. Read full article. (link)