It deters gaming because currently if you can affect the price of HIVE during the 3.5 day period, even by only 1% or less, you can potentially make a profit on the conversion (beyond what you are "supposed" to make from HBD being mispriced). Adding a fee increases the hurdle to where you have to move the price by more than the fee. The fee also represents a direct benefit to the blockchain. When trader convert under- or over-priced HBD they make a profit. The fee means that the profit is shared between the trader and the blockchain