USDT is bitfinex product and is listed everywhere. BNB is a chain that enables S-C, DAPS and binance still list ETH, NEO and other similar to their product coins. It is quite normal. The only thing that exchanges avoid is listing other exchanges main coins (BNB. KCS, LEO, HT).
According to what CZ said FTX tokens were the most traded tokens on their exchange. Volume=fees. And from business perspective this makes huge sense. And from business perspective FTX tokes were like another stable coin that is stable to BTC * multiplier. Just another USDT - bitfinex token that people like to trade with.