Did you claim that 50% deposit bonus, which appears to be opt-in and says:
Simply make a deposit of 200mBTC or more and you will be entitled to a 50% bonus on your qualifying deposit up to a maximum of 2000mBTC. Please note you will need to play through your deposit and bonus 30 times prior to making a withdrawal. Other terms apply.
or did they apply the restriction to your account, without you claiming that bonus?