I believe they do this because most critical softwares are American, and as long as the NSA has better offensive capabilities, it's better for the NSA if international defenses are low.

I don't think china or Russia really have good offensive capabilities, so as long as it is the case, this helps the US maintain some form of cyberweapon supremacy.

As long as china or small black hats don't do harm, they will not raise security standards.