In order to comply with the American Digital Millenium Copyright Act, Twitter has recently deleted tweets plagiarizing a joke made by an American journalist.
Although the impact of copying a tweet without acknowledging who the original author is might seem small, it can have a significant effect. Many accounts in Twitter generate money by promoting products and services, much like a television channel earns money from advertising. The reason why Twitter users follow these accounts is that, aside from that content, they can have access to tweets which they might find entertaining.
In the aforementioned context, copying tweets from another account could significantly alter the flow of followers, thus reducing the revenues obtained through the account in which the tweets where originally published. This is why it is not unlikely that similar enforcement actions will start taking place in the near future in other countries. Nonetheless, it should be noted that not all tweets are copyrightable, since not all of them meet the originality requirements set forth in most intellectual property laws.