Why it should be btc or mbtc? Just because we are in the crypto gambling industry so a site that accept crypto should not use fiat as the main currency to bet?
It is very common now, many sites use $ as the main currency although players are able to deposit with some different crypto.
Some sites do auto conversion (crypto to fiat) and some others keep the crypto but the they have an option to show balance in fiat.
All in all, every site are free to set their own rules/option about the denomination of the balance.
Damn right, I wouldn't bother contacting them to make it btc or mbtc as long as the site accepts different cryptocurrency as one of payment method when you deposit and in withdrawal. I wonder if he/she knows that some legit, trusted, reputable gambling site even made the currency showed is fiat but to deposit it accepts crypto and also in withdrawal so I don't see any problem to that but wonder why it must be mbtc or btc?.