Previous discussions here and here. When you ask Chinese officials why they feel compelled to control the Internet, the first thing you hear back is: It's not just China. No country believes in absolute free speech, on the internet or anywhere else. Many European countries censor neo-Nazi material. The U.S. and many other countries arrest people for child porn -- and until recently in the US, for animal-cruelty videos too. Australia's new policy on "filtering" dangerous sites is particularly ambitious, or aggressive, depending on your taste.
Naturally, Chinese officials well familiar with the outside world are the only ones who will make the argument this way. (Others just say: Stop meddling in the internal affairs of today's strong China!) But those sophisticates would conclude their case by saying: Each society defines what kind of material it deems harmful to its larger interests. We in China don't tell the Germans how to talk about neo-Nazis, and they should not tell us how to talk about Tibet.
[Update: courtesy of reader EG, this link to a report about the Chinese government's latest white-paper rationale for its internet censorship. Conveniently, it conforms to the summary I offer above! Eg:
Laws and regulations clearly prohibit the spread of information that contains content subverting state power, undermining national unity [or] infringing upon national honour and interests.]
The long message below and after the jump is from film director Tony Comstock, of Comstock Films and the Intent to Arouse site. His argument is about not governmental controls but what he sees as unaccountable private controls on expression by Internet companies, above all Google. He contends that, out of concern about being seen as pornography-mongers, they end up suppressing legitimate discussion of "sexuality, especially sexual dissent," which is his field of work.