“Contract terms and conditions vary between agencies; contracts to NASA and the military may differ significantly from civilian agency contracts. Civilian agencies and NASA are guided by the Federal Acquisition Regulations (FAR).”
Maybe that's for international protection as someone else reported in this thread. It seems that it's only in the US that NASA's works can't be copyrighted.
http://en.wikipedia.org/wiki/Copyright_status_of_work_by_the...