Because redeem doesnt burn FEI, any caller can mint and redeem multiple times in the same tx with the goal of arbing out the FEI - DAI pair.
SimpleFeiDaiPSM.sol#L105-L106
While FEI being tradeable for DAI is enforcing a 1-1 trade (FEI price goes up due to arbing, up to 100% + FEE), allowing the opposite swap is a easy target for arbitrageurs.
