Some exchanges have not, and have no plans, to give access to coins from bitcoin forks. Many of them are already worth a substantial amount of money, and likely will increase in value. Is there a legal basis for suing exchanges to get access to such coins?
...
The only 2 scenarios I can imagine, when you would have a ground to sue them is:
1) Exchange promised to credit fork-coins proportionally to customers' btc balance, but later changed their mind and didn't.
2) Exchange made it impossible for you to withdraw btc for unreasonably long period of time, i.e. overly long maintenance, your transaction was stuck as pending for weeks etc.
Other than that, they can do whatever they want, since, by not withdrawing, you didn't care about the fork.
It's kind of like saying: bank made profit on my 0% current account balance - I demand my share of it.