I'm guessing that the NSA website uses recaptcha, which is served by Google. Perhaps in order to comply with strict origin policy, they want everything on nsa.gov to be served from their domain. They seem to have a reverse proxy that proxies requests to google.com.
That's one plausible explanation, but in any case, even if my explanation is wrong, I doubt the explanation is interesting.
That's one plausible explanation, but in any case, even if my explanation is wrong, I doubt the explanation is interesting.