Exchanges aren't doing anything illegal, nor wrong here. It would be like suing a butcher because he doesn't sell horse meat. If a butcher only sells beef, that's his choice, and you're free to go to another to another butcher if you want some other meat.
Regarding fiat currencies now, within the Euro zone, it's pretty easy to exchange US dollars or Swiss francs, but it's much harder to sell Hungarian forints or Canadian dollars. Again, there's nothing illegal nor wrong.
You have a point and that is a great analogy.
That is a terrible analogy. Beef CANNOT TURN INTO horse meat. But "bitcoin" did turn into "btc + fork"
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?
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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.
Many exchanges have low withdrawal limits, so it can take a long time to withdraw the amount as BTC, while forks were taking place.
I imagine that whether there is a legal basis may be moot. If many persons pool together and sue the exchanges, the easiest way for them to resolve the issue would just be to release the forked coins.