What's up with that? It gets to he extremely agrevating. It discourages people from searching the free section and that in turn is a disservice to those needing to get rid of things asap or just hauled off.
It happens when they put $0 on an item. I have been bugging OU about not having a place for wants, trades/barter and fixing that $0 issue. Should have a "?" option in the price block for those that want to trade/barter or "*" for wants that sends those to the appropriate section, but they don't have those sections yet. That would deal with some of the $0 thing. Putting a popup in place if you choose $0 that informs you that it will be posted in the free section if you do that, should deal with the rest.