Every exchange trades tokens in their own ledger, not actual tokens. You don't own funds which you trade until you withdraw them from the exchange. Bitshares is different because it is decentralized. You keep tokens issued by gateways in your personal wallet, not in gateway's wallet. You actually own them. This does not remove a counter party risk though. In bitshares, you have an option to keep bitAssets, which removes a counter party risk completely.