Clearly, there's been a problem with automated systems redistributing jokes. Not sure if it's going to clear up those that just RT - i wouldn't have thought so as it's attributing the tweet to the originator.
A number of tweets have been deleted on copyright grounds for apparently stealing a bad joke. Twitter is Removing Stolen Jokes From Timelines Under DMCA [theverge.com]
Twitter, like many companies that host content from users, has an entire system for handling claims of copyright infringement. Under the Digital Millennium Copyright Act (DMCA), Twitter is provided "safe harbor" from copyright claims so long as it does not try to protect infringing material. Typically, claims concern embedded media like photos and videos, or they're for tweets that link to other websites that are illegally hosting copyrighted material, like movies. It's rarer for a DMCA request to involve the actual text of a 140-character tweet.