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This is wrong, most casinos bonuses are tailored to appeal tk the high rollers that is why casinos dedicate some staffs to attend to the VIPs who they give special bonuses and treatment to keep them in their platform. Most bonuses are arranged on such a ways to attract the big players, the rich, and keep them loyal to the casino. They are the people that dish out high wager which the business beed to thrive. I think you have to revisit your source to understand how gaming business actually work.
Bonuses are universal, in my opinion. They are different grade of bonuses, some are used to appeal to newcomers to the casino, while others are designed and exclusively offered to people who have some VIP degree within the casino, in order to keep them loyal and engaged with games, that is true.
Though, I personally don't know what the difference is between public bonuses and bonuses which are only offered to VIP gamblers, because I have never benefited from exclusive bonuses, I have only tried welcome bonuses and also some occasional deposit bonuses.