The primary answer to this is you. This is true for both casinos online and freestanding. In essence, they are counting on you as a player to go and spend your money at the casinos in the hope that they will be able to keep providing the feeling that you have a chance to win something as you empty your accounts into their coffers. This way they will be able to keep providing the entertainment and the activities up and running without having to charge for all of these free services. This is because you are so blindly putting money in a slot machine, or you are certain you are going to win the next hand of blackjack. So, no matter what you are really paying for all you are getting just not directly.
So, watch your money as it becomes theirs and be careful not to get in over your head.