For USDT withdrawals, they only support ERC20 network.
By the way, the transaction fee on ETH network isn't really expensive these days.
If you check the ethereum blockchain, you will see that USDT transactions are being confirmed with the fee of around 0.0001 ETH. I remember ERC20 transactions were much more expensive in the past.
Yes, you are right but most people still think that ETH fee are high and they avoid using ETH blockchain especially if we compare it with other chains. Anyways, bet105 should also give the option of Bep20 and TRX blockchain for withdrawals. Also, when we compare the different chains, BEP-20 transfers are almost free unless the sites themselves set some withdrawal fees in order to cover the maintenance costs.