Although it contains real names and real places, the film will justify itself as fictional making it harder to target for defamation / libel / slander.

Edit: Another way the film can get off the hook is by claiming the information depicted is based on the sources (books) it uses for forming the script. I can only speculate, but what matters is shaping the public opinion and they are doing it by leaking the script and expressing their opposition to it.

Another guess is they need definitive evidence before sueing the film/makers. To quote from their memo: "Although the film has premiered at the Toronto International Film Festival and numerous other media organizations have been shown review copies, the studio continues to refuse to show the film to WikiLeaks staff."