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.
Morally, the casino that did not ask you for proof of funds before accepting your deposit does not has the right to ask for proof of funds while withdrawing your money. Only that some casinos might be cunny, they might want to sit on the money, which is why they might be looking for excuses to make that possible.
But if you could prove this with a $500 monthly income, they will highly likely not disturb you since all they care about is how money enters into their pocket, not really about the welfare of the gambler. This is unless they are under a strong regulator that has some rules about earning and gambling. And in cases like this, a better alternative should be sorted out.