Because it looks like the attack is a government-based attack. As presented in the media, IP theft is par-for-the-course in China, but the only benefactors from reading the personal information of activists are those in the government, or those with strong government ties. This wasn't just a mere theft of intellectual property, this was a case of biting the hand that feeds China.

China needs money flowing into the country in order to sustain it's big growth rate. They've traditionally gotten it through dirt-cheap manufacturing costs and exports. Google's marketshare is small, but they're very well connected in the US and are probably the US company most likely to start a "China-free" product trend. If it takes hold, the US would get a little bit of extra leverage (something we've sorely been lacking lately)