But does holding BTC in their custody entitle you to their private keys? We don't own/control the private keys in an exchange's wallet. They are definitely obligated to release BTC we deposited into their custody. Past that, I don't know.
I think that's the key point here. The exchanges agreed to hold custody of your BTC and your BTC only. Any forks that may occur are a different matter, especially since anyone can fork BTC; and integrating alt coin wallets, especially hardforked coins is a non-trivial effort.
Worst case an exchange could argue that they charge a 100% fee on handling hardforks. Even traditional brokerages sometimes withhold parts of dividends in the form of a fee to cover their administrative overhead and the only thing that you can do as a customer is to switch to a different brokerage with better conditions.
Same holds true for cryptocurrency exchanges. If you are unhappy with their forking policy, just switch to a different exchange. If you fail to find an exchange that has a favorable forking policy, just withdraw the BTC back to your own custody. Unlike traditional assets the latter is at least an option with cryptocurrencies.