This is odd, since the definition quoted earlier implied it was the "lending institution" that asked for a reference, and the bank would supply it without necessarily informing the customer.
The casino MAY actually be breaking the law with such a request, since they are NOT advancing credit to the customer, nor are they RECEIVING a large sum of money in exchange for goods & services, and need to check the customer is "good" for it by looking at a bank reference.
It looks like "bank reference" is being misused here as a term for something similar, this being a "notarised ID". The bank is effectively "notarising" the player as being who they say they are, and are a customer with said bank.
This should NOT happen when players are able to supply a full set of ID documents, ONLY where players have problems supplying some items, and a substitute of equal quality is needed.
Casinos do seem to be pursuing players more in the courts, and through lawyers, so perhaps it is time PLAYERS took more of a stand, and started finding lawyers skilled in this field to take up issues on behalf of players.
Currently, if the casino stalls, refuses to cooperate with dispute resolution services, and regulators are weak, the casino can get away with choosing who to pay, and who to screw over, with little risk of much "come back".
There are quite a few players who would object to this because the bank would then KNOW THEY GAMBLE ONLINE, which would be an infringement of their privacy (to separate this leisure activity from their banking relationship).