It's manipulating the price of HIVE that is the bigger issue. Temporarily pushing up the price of HIVE means you pay less HIVE for the HBD. Also profiting from natural fluctuation since the collateral is only 2x. If HIVE decreases by more than 50% during the conversion period you still got the HBD but didn't post enough HIVE to pay for it. The blockchain then takes a loss. The fee helps the blockchain come out ahead on average. With no fee, the blockchain would lose on average. With 1%? I dunno, maybe. Under some situations 5% could potentially still be a loss. This was deemed acceptable.