Right. Exchanges usually handle deposits and withdrawals of coins independently of each other and can enable and disable them for a variety of reasons (suspicious activity, maintenance, hardfork, etc.). And usually trading happens internally to the exchange between individual accounts, from their POV whether a trade happens doesn't have to be tightly coupled to whether people move stuff onto or off of the exchange. They have fine-grain control over everything they do (it would be hard for them to operate as a mulit-currency exchange if they didn't since different conditions can affect different coins at different times).