Apple doesn’t know who the merchant is. The transaction data is encrypted on the device. Apple is not a bank and does not approve or decline transactions/payments. Only the issuing bank can approve or decline transactions. Most banks do not approve of the use of their money (credit card funds) for risky transactions. Another area of concern is converting credit card funds to a cash equivalent (gambling credits of some sort). Technically, this is a form of money laundering and banks can be very suspicious and concerned about the activity. After all TD Bank just got fined $3 billion several weeks ago for money laundering.
The obvious response is why did Apple Pay get declined, but when manually entered it went through. The merchant may use two different credit card processors that are attaching different merchant data to the different transaction methods. Just as a word of caution, banks have been known to shut down credit card accounts for risky behavior such as transactions with gambling establishments. My overall advice would be to use a debit card for gambling activity in the future.