I think the big point of confusion here was about the poloniex and bittrex accounts. The liquid steem in these accounts isn't really owned by these exchanges. These accounts hold the funds of all their customers. These exchanges accept Steem deposits from their customers and hold them in these accounts while their customers trade internal IOU tokens on the exchanges. The funds going in and out of these accounts are when people deposit and withdraw from the exchanges. The memo field is used by the exchanges so that the exchange can recognize which of their customers to credit for each deposit.

There's really nothing shady going on here, this is just standard method for cryptocurrency exchanges to handle customer deposit/withdrawals.