One problem is that multinational corporations are multinational with significant operations inside of China. I doubt that the NSA is going to be of much help in helping a US company secure its networks inside of China, and if that company has large numbers of Chinese employees in China, there are security issues with having US intelligence companies sharing sensitive US intelligence.

For example, the NYTimes has a bureau in Shanghai, and if they use a private contractor to help strengthen their networks, then I don't think that the Chinese government would be cancelling visas. If the NYTimes brought in the NSA, then I give the Chinese government five minutes before all of the visa get cancelled and the journalists get expelled.