Casinos will always do what is best for them at least they must profit from the process. Despite bonus been a way to foster interaction between a user and the casino it is primarily a way to generate extra income for the casino. With that I am okay but I am not comfortable with how difficult they make their wagering requirement to be. On a normal ground they are indirectly doing exactly the same thing gamblers will do if they continue with the none deposit bonus system. Which is ripping the other party.
Bonuses are a kind of advertising, they can't take them away and among other things they can't not use them, otherwise no one would have, as it were, the incentive to use a new casino. It's right what they do, otherwise no one would try anything else. It's pure competition
It is pure competition and part of the free market, and you are right when you point out bonuses are important for casinos to attract new gamblers to their website. In general, when people fell comfortable enough with a casino they like and have a good reputation, they stay gambling on it in a persistent way without giving other casinos a chance, specially those how are new in business and do not have much of a reputation or history within the market, without bonuses, there would be little those small casinos could do in order to get gambling volume onto their website and would inevitably get out of business.
Free market and fair competition within gambling industry is beneficial for all of us who enjoy spending some time on casinos, in the same matter free competition is beneficial to us in other aspects of our life outside and inside internet.