Kaszynski could have published a book. If none of the major publishers would have carried it, be could find an alternative publisher, or even self-publish. If it weren't sold in stores, it could have been photocopied. Remember that samizdat existed in the Soviet Union without requiring terrorism to publish.

No, he wanted eyeballs, and the best way to get those was to mail bombs. He wanted to terrify people into listening to him. His ideas weren't palatable on their own so he took a shortcut paid in human lives.