I think that is not the basic principle about licensing. For me, if a gambling site operates and accept money, that means they need a license in order for them to be regulated. The government can't tax a gambling site if they aren't regulated because they'll be unknown to them, so it doesn't make sense that there's no exemption on a gambling site and will be given a freedom to operate without a license.
Look at the title -
Why do they need a license if Bitcoin is not money?. The government says that Bitcoin is not money, so technically if a gambling casino accepts Bitcoin payments, that doesn't mean they are accepting money, that's why OP asks, why they need to acquire a license if Bitcoin is not money.
I think that casinos need to acquire licenses because they offer slots, live blackjack, live roulette and other games that are provided by 3rd parties. For example, when you run a bitcoin casino and have Evolution Live Blackjack, a user plays with cash on that table, so you need a license. If a casino offers only in-house games, like bustabit and freebitco.in, then I think you don't need a license, that's why they are without a license and have never had a problem with authorities.