Does it have to be transparent?
The casinos decide what to do with their own money and who to give bonuses to. They want to make more money, who are we to judge them?
The gambling businesses aren't charity foundations, after all. They don't have any social and moral obligations(sad, but true).
It is true that casinos are privately owned, not charitable institutions, but you are forgetting something very important:
even if you have a company, be your company 100%, when you go public and say the following:
" we will give 100$ bonus to 5000 people "
from the moment you said that, you have a moral obligation to fulfill what you promised, it doesn't matter if the casino is yours, you promised and you must fulfill what you promised, unless something serious happens that prevents you from fulfilling that promise so in this case you need to go public and say that you are no longer going to give that bonus for reasons X and Z.
now when you come to the public and promise 100$ bonus to 5000$ people and then you only give bonus to 500$ and don't tell people that only 500$ got bonus then you become a damn liar and it won't take even much time to make you scammers, because that is the attitude of a scammers, a serious and honest company would not do that kind of thing, a serious and honest person would not do that kind of thing. it's wrong to come and promote to give things to a certain number of people and then give a small number of people and lie that you gave to everyone
There's no point giving away money and bonuses to new players, who might play once or twice and leave the casino after a while.
bonus is a way to get people to play more at the casino, the problem is that currently there are too many casinos, so people can play today at casino X, not like it and move to casino Z. but this is a long subject , that I would have to write a lot about what I think about it, but in a few words the sign-up bonuses are no longer very attractive to keep customers in the casino for a long time, many new casinos do not have many resources when they are launched, so the only thing what is interesting are just the bonus and if there is a high requirement to claim the bonus then it will not make sense for the customer to remain in the casino