Since the exchanges are acting as custodial agents, they are holding bitcoin on behalf of the user, just like a brokerage does not "own" the stock but holds it on behalf of the customer. Whether or not the exchange plans on spending that bitcoin, if they do not give it to the customer they are stealing from the customer. Since coins from bitcoin forks are associated with the private keys held by the exchange, the exchange cannot argue that they did not "claim" them by taking no action, because they do own them, whether they wanted to or not.
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.