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.
I think the problem is more evident when they would also ask you for a statement of your account and not just proof of your salary.
Because from that it would emerge that you spent a part on "classic" expenses to support yourself/family, go shopping at the grocery, pay bills, etc...
At the same time how did you manage to deposit the same amount in the casino?
in any case it is something very subtle.
you can always prove the bottom line of your earnings, and I don't think any casino should declare it as "unreliable".
Obviously if they want to make a scam they can declare anything to deny a payment....
in any case even if you are violating the "principle of gambling for fun" this is not a good reason to close your account or deny you a withdrawal if you win