It helps them because they are using the service for covert operations. If they were the only ones using it, it would be useless to them. They did very cleverly position it as an instrument for dissidents and at the same time told the generals that this would actually be an advantage. On top of that the NSA is known to successfully target Tor users. If you are really doing something that is against US security interests, you would be mad to use Tor, that was all I was trying to say.
