Because as it is it can cost more than 3 times the price to put something in the auction house (for just 4 hours) than the item is worth: http://sadpanda.us/images/985672-YOU4H9F.png
Because as it is it can cost more than 3 times the price to put something in the auction house (for just 4 hours) than the item is worth: http://sadpanda.us/images/985672-YOU4H9F.png
It's an incentive to keep the price down somewhat. Fail to sell, listing fee gone. Sale, listing fee returned. If it's a bigger penalty, hopefully the more people would be enticed to keep the price at the 'sellable' level.