Even if the casino spends a lot of money on the promotion, they can get a return from the gamblers who lose their money so that is worth doing that.
The gamblers' bonus can invite more players to come, especially if the casino announces here as this forum has many members who often play gambling.
But it needs more responsibility from the players to prevent losing much money as if they follow all of the promotion, that will need much money to spend because we do not know when we can win the games.
Is it really worth it though? Depends on the marketing, even the same thing that one place does may not work for another place. For example casinos are known to do signature campaigns, and you could have two casinos both brand new, both have everything same, and they pay the same amount for everyone and for the same amount of weeks. The results could vary, it all depends on people. So, not all marketing could be profitable.
So that means you could spend maybe 50k dollars on a marketing campaign and you may get 30k dollars back from gamblers and they may end up leaving, and not that you are going to get it in the first day, it will take years for the last person who came from that campaign to leave. So all in all, marketing is not a profitable thing at 100% of the times, there are days when it doesn't just work and you end up losing money.