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.
That's a great answer, the license is a part of the 3rd party games, and is the way to trust those non-provably fair games. Some users like to play those fancy slots even if they can't verify the rolls, but they feel the game was fair for the simple fact that the site has a license.
Just to conclude the topic, a license is not a warranty for the users. We have seen a lot of licensed casinos who has a terrible reputation, but their marketing strategies is so strong that they keep getting new users, and more than 50% of those new users will end up having a bad experience.