There is also a substantial difference between the fee and EUR and USD or in BTC. If bitcoin goes up a lot, would the fees also go up? Actually, a casino should take care of the fee as customer service to the customer.
That would be fair.
EUR and USD are not cryptocurrencies so they won't depend on any blockchain network. Their transaction cost will depend on banks, merchants, etc.
Bitcoin is a cryptocurrency, has a blockchain and its transaction fee depends on network hashrate, waiting transactions in mempools, etc.
Gambling sites can not control transactions fees from banks or blockchains, miners but they can waive fees for customers. Recent years, less and less gambling sites do so for customers.