I have seen other casinos/books return the difference to the customers if they overcharge customers, I reached out to their live support and discord many times but with no luck.
What did they say? Did they acknowledge that only a small part of the withdrawal fees go to the miners, or are they trying to conceal that fact? There isn't anything illegal in doing that, but they could lower it at least.
I had a chat with a live support from a gambling website whose name I am not taking here. I told them that what they charge, they should fully use that amount of btc in fees instead of keeping a part of it with them, as it stops the transaction to go through faster than the time it takes with the fee they send it. They replied that the fees are dynamic and change during each of my withdrawals because of the need of that time. They assured me that none of my withdrawals will take more than few hours to confirm.