Field service is a term that is normally applied to engineers. The Watchtower society's funding had come from its literature selling operations. Anyone familiar with cold canvassing, will be aware that the terms they use are straight out of American cold selling manuals.
Now the tower has had to stop asking for a fixed price from the public in order to maintain their tax free charity status. The members still have to pay for literature, even the magazines they give away. The congregation has to pay for all pamphlets ordered. So the presses keep rolling and the dollars pour in.
The congregations and kingdom halls do not belong to the Watchtower society. The Watchtower society is first and foremost a commercial publishing company that specialises in cold selling. They trick unpaid workers into distributing literature at their own expense, with the false promise of a reward that has never been delivered.
All this has nothing to do with Jesus or Christian ministry. Its all about the Watchtower societies worldly empire and their old testament, Hebrew desert god, Jehovah.