Yeah, I get a bit confused by that also. Googling around a bit, I found this Steemit post helpful. From what I understand (which isn't much), it's similar to any other exchange in that you're trusting whoever runs that asset to play fair. If they go belly up, you may be out of luck, but the smart contracts of bitUSD and bitBTC can provide some security as long as the value of BitShares doesn't go through the floor.