Great question. Just a thought, two things are going on here. First is the US gov't lets the private corporations defend themselves because it would cost much in tax dollars to protect them. Why should US taxpayers protect private company's intellectual property? A real point of debate. Second, they don't want to disclose our current level of technology, our tactics to combat it, and we want to see what they are looking at.
Then again, the Russians or the Chinese could have already hacked into the latter and they know anyway. What we do is that it is considered a weapon of warfare that could cripple a country's economy, infrastructure, and probably the military.