I don't think it's necessary to demand that the casino sites show proof of their backup because I'm sure they keep the funds in their cold wallets and don't keep any exchanges, so that issue should be separated FTX with the casino site, keep in mind to play on a trusted casino site
They're not obliged to do that. But just like the given example of bustabit, bustadice also has the same feature. As I've been part of its campaign before. They're transparent and they have numbers on their website of how much they've got in their bankroll.
The amount of bankroll they have is visible on their website. So if someone wants to look at how much they've got, you can freely check it. But this is not for most casinos and to demand them, it's up to them if they'll consider the recent situation and issue about FTX.
If there's no protest or demand from most of its loyal customers, they don't have to do that.