BitUSD is a prediction market on the consensus value of BitShares vs FedUSD and that is all it is. Consensus value will track the value of FedUSD without ever having to store FedUSD with any trusted party. Anyone who doesn't invest according to where the consensus is going to move will lose money.
What money they'll lose?
They can lose only BitUSD. They can't lose FedUSD because you have no access to their FedUSD. The value of BitUSD is absolute zero if losing BitUSD doesn't automatically inflict losing the "real" stuff (FedUSD)!