I am surprised it hasn't happened earlier. Not a great idea to have a foreign country potentially control your vital infrastructure if you aren't always on the most agreeable terms. Not saying the US and China will go at it, but also they aren't always the best friends.

That said I know people who have worked in the NSA and they have said that even with Linux they have back doors in there, they just haven't used them unless absolutely necessary.

I guess it is because as soon as they are used they can be patched out fairly quickly so they have to use them wisely.