i think it doesn't matter. because the casino only wants the reputation of the player and the casino to remain good.
all I know when the casino asks for proof of your salary is just to prove that you really put in $ 500 of your job salary. because what the casino fears every time you put money into the casino with the same amount every time is afraid if the money is the result of money laundering or corruption.
so if the casino has received proof of your salary they will be fine.
I don't think there's a casino will accuse their users get money from laundering or corruption when the gamblers only spend $500/month, it's small amount for the casino since they can make much much more higher than that. Also when you're deposit with the same amount every month, the casino will recognize if it's from your salary since most people get paid every month.
I believe the casino just want to collect more KYC in order to more know about the gamblers, there's no any reason except that.
Doesn't makes sense at all for me.
If they do , it's probably an unfair casino tha looking for reasons to hold customer funds which yeah it's a scam attempt by chance. Or if the casino legitimate enough to ask that ... could be mean that there's a request from the regulator and the casino can't do much to refuse it.