Bonus hunting (or bonus abusing) is a term used to describe the activity of actively and systematically trying to use online casino bonuses to make money. Casinos often include clauses about bonus abusers in their Terms and Conditions (T&Cs), allowing them to withhold the winnings of players suspected of being bonus abusers.
Bonus abusers can be costly for online casinos. They try to use casino bonuses and play in specific ways to gain a statistical advantage over the casino. And because casinos base their entire business strategy on the statistical advantage they have over players (the house edge), it's understandable that they want to protect themselves.
Casinos often use anti-bonus abuse clauses in their T&Cs retroactively, confiscating winnings after the wagering requirements have been cleared and the player has asked for a withdrawal.
If a player has received a bonus, they don't expect it to be taken away from them. They expect to be able to play with this bonus and withdraw money if they manage to win. If a player is not a bonus abuser, they probably don't even know that something like bonus abuse exists. If a player is a bonus abuser, the situation is a bit different, but if they do receive a bonus, they still expect to be able to use it and possibly win some money.
If a player uses their own personal information to create a casino account, doesn't have duplicate accounts and follows all of the rules in the T&Cs, they should be able to use a casino bonus that has been given to them in any way they want.
To combat bonus abusers, casinos should implement additional rules in the bonus T&Cs in order to limit the ability to abuse casino bonuses. If the casino does not want to take any extra measures, it then should try to identify bonus hunters when they create their account. If some bonus abusers still go through this procedure and the casino identifies them based on their playing style, displaying an internal message and ceasing to offer bonuses to these players may be a way to go.