I have not been to a meeting in twelve years but as a former witness of thirteen years I was under the impression that the "free" literature was "free" to those in the world. Didn't the society do this for tax purposes of some sort? Or was it so they can preach in buildings since they were not "selling" anything. It's been a long time so I don't remember why it was done but to the best of my knowledge publications were "free" to the outside public but witnesses were still expected to donate.
The part I never understood was as a child I would watch my parents "donate" money for the publications they used to preach with than collected donations for them and put the money they got right back into the contribution box like many other witnesses. I always found it odd that these people would buy the literature with their own money than collect money for the literature they sold and give it back. It's like paying the society for the priveledge of going out preaching for hours.