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.