Ok, suddenly the casino you're playing asks you for proof of income to be able to withdraw your winnings and to keep your account in good standing, your bankroll every month is $500 and you provide proof of salary that you are making $500 net every month, will they accept that proof will they accept that you are putting all your salary for gambling, will this violate the principle of gamble for fun, because you are working to gamble based on your proof income.
Maybe the better question would be, why ask that concern in the first place?
I don't see why an online casino will ask for your proof of income much especially with the amount you have given as an example.
What basically they will gain if we are able to give proofs that we are making around $500 a month?
Peace of mind?
Assuming that the amount is too huge for a casino, and the casino is suspicious of our fund so they will asked us where is our fund coming from supposedly to comply for AML regulation. It will give them valid proof that our fund is legit when some authority look at their transactions and sees that we are depositing huge amount of money. So less headache for them because they will be spared of being accused harboring money launderers.