I think in the past they are allowing, but now that they see the risk, they don't allow it anymore. The risk here is that, once they make a deposit, they can put a little fee so it will not confirm right away, gamble the credits and if that bet will lose, they will just double spend it, that's so clever from the gambler's side so gambling sites have to also adjust.
I guess Russlenat is referring to Sportsbet since they're one of the few sportsbook/casino that instantly credits your bitcoin deposit with no confirmation and at the same time they require confirmation on withdrawals.
It could be abused.
I agree it can be abused but these casinos already have a solution against it like they'd automatically flag your account and put some heavy restrictions where you can't do anything with the account until you contact their support.