The book was published in 1995 [1,2]. Free speech protection for source code under US law wasn't decided until 1996, in Bernstein v. United States [3].

[1] https://en.wikipedia.org/wiki/Phil_Zimmermann#Arms_Export_Co...

[2] https://www.eff.org/deeplinks/2015/04/remembering-case-estab...

[3] https://en.wikipedia.org/wiki/Bernstein_v._United_States