Unfortunately, lots of places do it because they know that if you can't withdraw you will probably be tempted to play some more, but it is borderline scammy behavior. Any reputable casino should allow you to withdraw your winnings whenever you want them.

Obviously nobody is going to keep their whole bankroll online, available for instant withdrawal, but there's no reason not to keep enough available online to service reasonable withdrawals.

At Just-Dice for instance we have a bankroll worth around $1.5M but only have coins worth something like $40k available for instant withdrawal. If you win more than $40k's worth you'll need to wait for up to 12 hours for the hot wallet to be refilled.