NASA web developers obviously not rocket scientists, how silly.
nasa.gov
This is silly. If you try accessing http://nasa.gov/ it does not open up, unless you add www.
nasa.gov
Short version: They have decided the cost of doing so is not worth it. I could not agree more - sure, redirecting example.com -> www.example.com is technically straightforward, but setting up a highly-available cluster (which NASA would certainly require) is not free.