US copyright protection is not available for any work of the United States Government, including those works produced as part of a NPS faculty member's official duties. For more information about "official" versus "personal" works:
Wikipedia has provided a simple explanation for the historical reason Government works have no copyright. Here is a quote from the entry on the Printing Act of 1895.
The Printing Act of 1895, was a law designed to centralize in the United States Government Printing Office the printing, binding, and distribution of U.S. Government documents. The Act revised public printing laws and established the roles of the Federal Depository Library Program (FDLP) and the Government Printing Office (GPO) in distributing government information. The act also assigned leadership of the program to the Superintendent of Public Documents, who would be under the control of the GPO,
It is the policy of NPS that faculty, staff, and students of NPS abide by the following laws and instructions with respect to all use of copyrighted materials while at NPS.
Permission from copyright holders is often needed when creating course materials, research papers, and web sites. Similarly, thesis authors need to obtain permission to use copyrighted materials.
You need to obtain permission when you use a work in a way that infringes on the exclusive rights granted to a copyright holder (i.e. outside the boundaries of fair use).