It would not surprise me in the least if the United States government wasn't working at ways of hacking, controlling, etc. websites,
Uhm. We know they conduct security experiments and attempt to subvert common web services. How? Because there are DoD employees who submit security patches to open-source projects related to them!
and I don't doubt that if they knew of a way to hack into the Chinese power grid they'd jump on the chance.
Why? There is no state of war between China and the United States. Neither country gains anything from a scrap. The United States certainly doesn't gain anything by starting one.
Considering all the various things they got up to in the Cold War, after all... that's just what governments do.
Not relevant. This isn't a cold war. "Hurr China's gonna eat everybody" is misguided. If anything, the Chinese are likely to be among our closest allies in the twenty-first century, because they are on a path toward liberalism that is unlikely to reverse itself (without significant damage to China's future economic prospects, which are understandably more important to them than their political structure).
If anything, China's being sloppy about it by getting caught.
This shows a fundamental failure to understand how network security works. If you can't tell where data streams from your own network are going, you have genuinely incompetent staff. While penetration is often doable, covering your tracks is a far harder task. Chinese elements (there is no confirmation that there is a unified Chinese decision to attack Google; it very well could be a case of the left hand not knowing what the right is doing) clearly relied on the assumption that Google, like the other companies mentioned in the various discussions about this topic, wouldn't start a shitfight. They were wrong.