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. BUT...What if they were not supporting the currencies for the trading on their exchange, but did indeed hold onto a wallet for each account and accept the incoming forked currencies.
That would be illegal. Simply saying you won't support the coins on the exchange, but taking the coins due to the owners would be a huge issue.
So it would require someone blowing a whistle or some sort of audit to discover such activity.
There is a lot of money at stake and I refuse to believe exchange owners are just letting that go into oblivion. That is not the way financially savy people work. More likely they are holding the coins, trading on them, and will give them to the owners when they have lost the value they started with. But hey, I could just be drinking the conspiracy cool aid for too long to believe in people doing the right thing when it comes to vast sums of money at stake.