That would raise a variety of other serious issues. Besides the ethical problem that just because someone else does something bad, doesn't make it ok for you to do that bad thing.

I could list several important reasons why "hacking back" would be a bad idea, but I'll just stick to the pragmatic "people who live in glass houses shouldn't throw stones". We have a much larger surface area and much more to lose if we get into that kind of tit-for-tat. Instead, the US is building a case that China is an aggressor, and is responding in other ways that will apply more pressure. For example, tariffs on Chinese exports.