JimmyPage,
The Society charged 3% interest for about three decades, then increased it, and then lowered it back again. There were two reasons for the interest charge. The original Kingdom Hall fund was from an estate that was willed to the Society to manage ... it was not Watchtower money. The estate set up the fund to finance Kingdom Halls, and required the 3% to cover the usual average cost of inflation. The Society did not make money as they had to return the money to the estate fund and loan out to other new Kingdom Halls, or for remodeling and expanding existing Halls.
Eventually, inflation out-paced the interest charge. I recall inflation hitting 12% and more in the late 1970s. In those cases the funds used for Kingdom Hall construction were falling behind - losing money. So, the Society eventually increased the interest charge to 6%, which was still too low. Also, the Society expanded the Kingdom Hall building fund by seeking additional donations and loans from JWs. Eventually, the interest charge came back down to 3% as inflation cooled down in the mid-1980s onward.
I do not know of the Society eliminating the interest charge as claimed by MinisterAmos. If they have, they are losing money, and they are foolish to do so, because in the last 15 years or so, the economy has inflated at about 3%. The Biblical injunction against interest charges was not a prohibition against building and construction loans or against Christians making a profit. Rather, it was a prohibition of charging interest to a "brother" who was having personal financial troubles. It was intended to help Christians to be merciful with a brother in hard times, and not take advantage of him. When I was a JW, I went through such hard times and was loaned money interest free by kind and generous fellow JWs.
The Watchtower Society does not make a profit on Kingdom Halls. Aside from the low interest barely covering inflation, the local congregation owns the Hall. The Society is named as beneficiary in the event of the demise of the Hall, so that if a congregation turns "apostate" it can never again seize the Hall as was the case in the Bonham, Texas lawsuit. However, if the local congregation sells the Hall and builds another, they keep the funds toward that project, not the Society. If the Watchtower Society starts making a profit on anything, they can be disallowed their non-profit status and be forced to pay income taxes. As much as we disagree with the Watchtower Society, and as bad as they have proven to be, they are not entirely stupid ... and they are not going to take that level of risk, especially since they do not need such a risk.