I think if you’re looking for a logical answer from first principles, you won’t find one. It’s more that the legal system runs on precedent, and a book fits far more squarely in the fact patterns of previous First Amendment cases. Likely the source code case would end up with the same outcome, but it doesn’t hurt to make it more obvious.
Legally speaking, it didn't really matter. But symbolically, having the feds argue that a book constitutes "munitions" would be bad optics for them in a way that is more understandable to the average American, compared to more legal arcane arguments about software having 1A protections.
This was a time when we were happy when they managed to get congresspeople to understand that the internet is not like a truck, but more like a series of tubes.
I think anchoring it to something old school like a book was a good call.
