This may seem like some form of censorship taking over Twitter, but the company only mentioned those of "historical or cultural reasons" like the ban of pro-Nazi content in France and Germany; so it's not clear whether Twitter will also handle similarly with tweets that potentially lead to events such as the UK riots last year. Even though Twitter didn't elaborate further for Reuters, there is one reassuring line in the post:
One such country is most likely China, and back at AsiaD in October, Twitter CEO Jack Dorsey told us that there's simply no way for his company to work with the Chinese government (you can watch him answering us at 38:17 in the video -- courtesy of All Things D -- after the break):
"Some [countries] differ so much from our ideas that we will not be able to exist there."
There are obviously many factors that add up to this sour relationship, but the contradiction between China's strict internet monitoring policy and Twitter's core values is most likely the biggest obstacle. And of course, the Chinese government would favor its home-grown tech properties, anyway. That said, several months ago, one of the country's largest microblogging services Sina Weibo was criticized by the authorities for not censoring fast enough, so it's obvious that it'd be even trickier to work with a foreign company that sees things differently. Things are unlikely to change any time soon, or ever, unless China relaxes its policy.
"The unfortunate fact is we're just not allowed to compete in this market, and that's not up to us to change. The person to ask is trade experts between both governments, but at the end of the day we can't compete. They (Chinese microblogging platforms) can compete in our markets, and we're certainly interested in what that means for us... We would love to have a strong Twitter in China, but we'd need to be allowed to do that."