The Guardian ran everything by the US government first before publishing. Every responsible journalist does. They're looking for a response or comment to the story, to make sure they didn't get anything wrong, and to see if there's a any reason to withhold details (a reason beyond "It's classified so don't").

In fact, in Greenwald's book No Place to Hide, he describes how they held off publishing for several days while waiting for a response from the government. Greenwald was so frustrated with the delays that he considered quitting the Guardian and going on his own. I think that's the idea that eventually turned into The Intercept.