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.
If they manage to put a backdoor in any software or hardware you use you are affected. They could spy on you and trade that with your government or just lose the keys and now anyone can do it.
It infuriates me the NSA actively works to undermine American security. Their brief is to protect us, not plant backdoors and then lose the keys.