Bonus is not meant to become profitable to the casino. They are gambling on the marketing hoping that they will attract more users but that doesn’t that a player conversion is guaranteed.
They are some sort of profits to the casino in the end because they are getting more bets from you by providing these bonuses.They have marketing expense but also gain a lot from it in the end and for players also if they get some good wins out of it they are happy for it.
With the amount of casinos that are around, all casinos need to do something and offer promotions, bonuses, contests, … You always need to trigger the interest of the people to not only attract new players but also, and more importantly, retain your old players.