You are not the first to point out this perceived problem: http://wiki.nasa.gov/cm/blog/nasadotgov/posts/post_130686081...

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.

This is silly. If you try accessing http://nasa.gov/ it does not open up, unless you add www.