I'm no policy expert, but it seems likely to me to be a demonstration of capability. The Chinese government has been arguing for awhile now for their right to "Internet Sovereignty", the ability to impose Chinese policy on Chinese users of the Internet. This attack seems to demonstrate they also feel the right to extend that sovereignty into other countries, to block services like GreatFire they think are harmful to their sovereignty. This attack is them showing the world they have the capability.
Lets not forget the context that the Snowden disclosures brought into light. Borders and diplomatic relationships has been systematically ignored when it comes to developing and deploying offensive capabilities on the Internet. Sovereignty does not seem to exist in nations mind when developing malware, breaking into critical infrastructure, stealing information, or causing sabotage online.
More detail on China's Internet Sovereignty theory: http://blogs.wsj.com/chinarealtime/2014/06/23/chinas-lays-ou... http://www.huffingtonpost.com/lu-wei/china-cyber-sovereignty...?