It becomes too costly when I just have $20 to withdraw and the fee is worth more than $7.5, I will get almost nothing on my hands.
You don't have to withdraw your funds when the mempool is full and there are tens of thousands of unconfirmed transactions. Just hold on to your funds a few days. Use weekends for withdrawals for example.
They shouldn't pay our fees, there is no need of this but I think that fees shouldn't change everyday, maybe weekly updates can be a good choice.
Other casinos pay the transaction fees for their players. For example FortuneJack and Sportsbet. I never paid a cent when withdrawing from those platforms. If players are charged with paying the network fees, they should at least be given a chance to adjust the fee rate how they see fit so they don't end up overpaying.