Twitter has not been nearly as eager to enter the content policing game in other situations. Like many other major companies, Twitter has long insisted it is not a publisher but a platform.
The distinction is an important one: publishers, such as the Guardian, bear a far greater degree of responsibility for what appears on their sites. By remaining a platform, Twitter is absolved of legal responsibility for most of the content of tweets. But by making what is in essence an editorial decision not to host a certain type of content, Twitter is rapidly blurring that line.