Well, it's true, gifts, giveaways, bonuses, special offers, bonanzas, perks, calling it anything, as long as it offers some kind of benefits to the customer, is really one effective ways customers can be made to be loyal to a casino.
Speaking from my own personal experience, there are still some casinos I can't stop using, simply because of the small perks, offers and bonuses I get there, it makes me sometimes feel like the casino is giving to me, I should also give back to them too - this is the indebted part you mentioned.
As a business, they must have income and outcome and they have to balance their marketing budget and income as well as profit from their income in order to keep their business running as sustainable as possible.
They can use a budget for marketing and stop it anytime but if from marketing, their business get benefits from it like more new real users, more income, they will have some reasons to assess their marketing and consider to expand it longer and longer until something change with marketing result.
Being loyal with a business by actively using their products will help users maintaining their ranks, levels as well as related bonus given by loyal programs from the casino. It is part of marketing and wise strategy to keep a business running well. They must maintain their current users and at the same time, reach to new communities and try to get new users.