The exchanges agreed to hold custody of your BTC and your BTC only. Any forks that may occur are a different matter
I don't think this argument should hold much to water your position, although you made and had a soft landing of your post in the end. Thus, talking about forking and holding of the coin itself, I think it depends on the exchange because I noticed that certain exchanges especially, coinexchange does release fork coins.
Also, I think that should even be the case naturally, that exchanges should also have it as a duty to deal with the properties of the coin as if it were the coin directly; that is ,every proceeds of the coin accruing from the coin.
Take for instance, coinomi do have provisions for fork coins or tokens. All you do is to update your wallet and receive it direct to same wallet. I think it should be nice that exchanges does a thing like that whether it is outrightly a legal obligation or not.
I could imagine people pooling together to sue exchanges if this argument is viable.
This would have obviously been the case but this is not a physical business that would have necessitated converging interested or affected parties. Moreover, it is a decentralized business where people of anonymous identity are involved.
Were it to be like stock, where it is physically situated in a particular territory with a centralized office, then adjudication would have been the order where principals contravened.