PGP (and all encryption schemes that offered key lengths greater than 40-bits) were banned under the State Department ITAR regulations.

These are the same regulations that prohibit the export of 3D model files over the internet if the model is a weapon or a part of a weapon.

The 3D models are banned because they are "technical documentation" (blueprints, manufacturing information). I would argue that the source code to encryption (a "weapon" according to the regulations) also counts are technical documentation. Therefor I think that printing the source code was just as illegal as sending the source code (or compiled software) over the internet.

Try exporting the blueprints or software source for a Gen III+ night vision and see how quickly you are picked up by the FBI.

I can't help but think this whole book thing was more designed to shame the government into letting it be exported, or for a judge to declare that the regulations were unconstitutional, or the guys who did it were misinformed about the law.