I think there were different policies depending on the venue.
As best as I can remember:
The WTS tried very hard to get the city or venue owner to drop the parking charges for the conventions along with concessions.
Most places dropped concessions readily enough but a few places still insisted on parking. The majority of venues that dropped the parking charges was where the WTS would issue WT parking passes to be purchased at the hall prior to the convention to gain entrance to the parking lot at the venue.
Though it was never stated outright, it was assumed by the publishers that the WTS was being charged by the venue or city for parking based on estimates of attendance and was therefore passing the cost to the publisher and as good publishers we had no problem with ponying up the dough. We, of course, assumed wrong. As the article that blondie posted shows, it was all a scam. The WTS had negotiated some of the cost of parking into the rental price or negotiated free parking altogether, then turned around and charged each of us $10 to park at a parking lot that had already been acquired in the price of the rental of the venue. Once it was leaked, by an ethical brother no less, the jig was up and the plug was pulled on the scam. Since then parking has been free at the majority of WT conventions.
Someone mentioned that the brother who blew the whistle was DFd. Is there proof of this? I don't remember hearing about that.
edited: Duh. If I had finished reading the article I would have seen it. Doh!