I think it is in the exchanges best interest to try to get themselves setup so they can issue users with BCC otherwise if they haven't already, a lot of users will be moving their bitcoin out of the exchanges into their own wallets.
You should have your Bitcoins stored in a wallet whose private keys you exclusively control regardless of what the exchange you use says will do.
But it won't affect them much, it's only temporary and users will be returning to exchanges soon.