Gambling sites these day are not falling for it and which is what that matters.
At least 1 confirmation is very important.
Before someone go into a business, such person need to know more about that business. They might know about gambling already but how about bitcoin double spending. No hard to know and not hard to avoid. Just wait for at least 1 confirmation as a casino.
If no rules about withdrawal than making people to withdraw without any gambling rules like minimum amount to win before gambling, the withdraw can be allowed only after 2 or even 3 confirmations.