Casinos are a business like any other and want to make money, so if a user is losing money and giving profit to the casino, they will want to keep hin in house as much as possible.
So yeah it makes sense to give this user more bonuses and rakebacks, because at the end he might still lose more and the profit keeps coming. But would you call this type of gambler vulnerable? Maybe he has fun gambling even if he loses money when the day ends.
The most important thing is to keep your customers because the house edge is enough for even some slow profit.
You are on point, bonuses are just way to keep the gamblers In the game... let's imagine that casinos doesn't give out free games to their big stakers this might be a discouragement to them, they keep the minds of gamblers enticed with these free games and when it's exhausted due to a loss it would prompt the gambler to keep staking... only those that are disciplined can avoid such mental trap