There are two things where Cryptocurrency based gambling sites are still complying with the law and I can consider what they are doing as a loophole in most gambling laws.

1. Location - As you can see most of the Crypto-based gambling sites come from a country called, Curacao, it is a constituent country of the Kingdom of Netherlands and they are known for having the most easiest way on obtaining an online gambling license, aside from that the license offers a lot of benefits and that is involving the establishments of necessary accounts and sublicenses needed to operate your own gambling site. And of course in order to make this valid your operations must be located in Curacao. Just take a look on both License validation of FortuneJack and Bitsler.

They are complying through gambling laws as governments are only strict when handling and providing licenses to local-based gambling sites they come and operate on their own country, and aside from that their own citizens are not prohibited from playing foreign-based gambling sites as they deemed it legal, even though it looks unfair for local operators.

2. Money - With crypto based gambling sites only accepting Bitcoin and other cryptocurrencies they are actually not touching any real Fiat currency of any country which means the money of their own country are not flowing in and out directly through them. And since these cryptocurrencies still have not any kind of legal or illegal standing (on most countries) they can freely run their operations without being bothered by these countries.