There's nothing bad in the casino providing the crypto balance in fiat cause it will even help the gambler to manage his budget very well rather than gambling more than she can afford to loose.
It is true that gamblers will be able to manage easier when he/she knows the value of his/her crypto in fiat rather than the crypto itself. Well, it's up to gambler's choice whether he/she likes it or not but if the gambling platform prefers to show the value of crypto used in fiat then the gamblers will have to use it.
When the BTC convert to fiat you gamble against the crypto value.
I will wait to official replay from them. Maybe they do have option to bet in BTC