These exchanges are only going to credit users coins on one chain if there is a split right? So the exchange would get to keep any "duplicate" (etc-like) coins for themselves. There were likely many users planning to withdraw before Aug 1 for this very reason and now they cant. They should have announced the cutoff date well in advance.
THe bittrex has disabled the bitcoin service but not for another in my knowledge. Remember the bittrex exchange site already made an announcement for the users to disable the service until the soft fork will have finished. But what do you mean about there is a split? It doesn't make sense just only to credit the users if there is a split. Those voters are voting BIP91 caused by a split. Are you feel worried about the split?