When a company gives you deposit bonus, it certainly comes with a binding terms and conditions. Otherwise, anyone would deposit 1 bitcoin to that website and onec the bonus is paid, they would withdraw the total amount. Will it help the website in any way? That's why there are many types of terms and conditions attached with such bonuses.

The most reasonable one is, that you have to play a certain number of games before you become eligible to withdraw the amount. However, based on their business situations, additional conditions are applied.

Also such deposit bonuses are not applicable for gambling only. There are many forex trading websites offer such bonuses. Where you get $50 in bonus but the minimum to withdraw is $100. Otherwise it will become an endless circle of deposits and withdrawals, ain't it??