Also Twitter Inc has become a part of the censorship trend or "War On Truth" movement. But as for Twitter there are workaround for their filtering.
The filter is basically said to be based on countries. So for instance is the US uses the NDAA, and enforce 1984 kind of style censorship, and your a US citizen, you might want to change your country setting from United States to let say Portugal in stead. Worldwide is also highly recommended. According to sources, Twitter will keep it that way as the Corporation needs the twittering to go on to sustain it. And here is of course the key to it all. Twitter Inc needs twittering as this is their cash cow.
Meanwhile the US Government and others wants certain facts, news or information's to be censored. So there is a conflict of interest here and quite a big one as well. If something really big happens and Twitter Inc is caught red handed by withholding information trough censorship, their stocks will plunge.
Learn more about Twitter and censorship from this great article or read about it at the well censored Wikipedia's article on censorship by nations and Twitter