This isn't "Free as in Freedom" it's "Free as in Promotional".
FTFA:
Printed books will continue to be available for purchase through the NAP website and traditional channels. The free PDFs are available exclusively from the NAP’s website, The National Academies Press [nap.edu], and remain subject to copyright laws. PDF versions exist for the vast majority of NAP books. Exceptions include some books that were published before the advent of PDFs; books from the Joseph Henry Press imprint; and in cases where contractually prohibited, such as reference books in the Nutrient Requirements of Domestic Animals series.
So, you can download them to your computer, but you can't (legally) make a copy for your friend... This isn't the free as in "land of the free" that I grew up learning about... seems like a trap to me. "How did you know that without ever buying our book or downloading our PDF? You must be a sea faring rapist and murderous theif!"
Let me know when it's released under a CC license, then I'll think about downloading it.