This is such a great feature (help) for all of the gamblers because withdrawal fee is sometimes a headache for all of us, especially in Ethereum that happened few months ago even though it's not related on gambling.
But this is not surprising at all because all of the gambling sites that has been mentioned by OP were all famous, big, and reputable for providing such good services in the world of gambling when it comes to cryptocurrency and giving aid such like free withdrawal fees is just a small simple gratitude from them to their clients for always appreciating their services.
There's always a lot of ways to prevent ourselves from paying huge withdrawal, choosing a coin with less withdrawal fee would always help. For instance, I've been using XRP and Doge, I don't feel the withdrawal fee and it's faster as it's network is not congested.