Those stablecoins are also cryptocurrencies, which, if there is any survey conducted, will be associated with Bitcoin, and a total score will be given. The regulation of casinos I don't think it's as a result of stablecoins, but because crypto is getting more popular and it's also considered finance, so they can't stop but regulate it and get tax from the casino and also monitor their citizens' gambling activities.
Stable coins are pegged to fiat, and regulated there are by the government. It's just as good as fiat, although there fall under cryptocurrency, or better still digital dollar, that operates through crypto empowered networks. And it's presumably the only set of crypto that could easily get regulated. If players decides to gamble with USDT, it doesn't in any way remove the fact that they're playing with a cryptocurrency, because it's still working under the crypto network and recorded in a ledger.