Exchanges have to operate this way to make many thousand transactions per second. No coin can do that from wallet to wallet. Within the security of the exchange, the exchange creates a virtual environment to make transactions. The exchange acts like a middle man to handle the transfers and to keep track. Since they are not trading SBD, the question is what they are trading and what are they going to show if they get audited? Poloniex is in the same situation.

https://steemit.com/teamaustralia/@cryptoslicex/steem-monsters-card-game-built-on-steem-review