Not always. However, even if you can’t cash out the value of your bonus, you will still be able to cash out your winnings - provided you have fulfilled the terms and conditions.
Whether or not you are allowed to cash out your bonus plus your winnings depends on the type of bonus you claim:
Non-Cashable Casino Bonuses:
Non-cashable casino bonuses do not allow you to cash out the value of your bonus as well as your winnings. For example, if your bonus is worth $100 and you win $50, you will not be able to withdraw your $100 bonus as well as your $50 in winnings. Your withdrawal will be limited to your winnings only.
Cashable Casino Bonuses:
Cashable casino bonuses allow you to cash out the value of your bonus as well as your winnings. Unlike with non-cashable casino bonuses, if your bonus is worth $100 and you also win $50, you will be able to cash-out the sum-total of $150.