I believe the idea of individual JWs being called publishers is, not because they publish books and magazines (the Watchtower organization does that), but because they proclaim information 'publicly.'
On dictionary.com this is the 4th definition of publish ...
4. to announce formally or officially; proclaim; promulgate.
The most common use of 'publish' is to make books etc, but to announce something verbally is still a way of publishing, which is what the individual JW does.