The one I worked at had a ramp fee. The reason for this was simple: Contract Fuel. Many operators are looking for a way to decrease costs (naturally) and contract fuel has a lot to do with this. Colt, Mariah, Western Petroleum, etc are great tools. However in recent years FBOs have had to decrease their into-plane rates. Some contract providers are only paying 10 cents per gallon for into plane rates. This proves to be an interesting problem. Get rid of the contract fuel, and loose business, or charge a fee.