The reason why some sites don't state that is because it should be common sense that as a USA resident you are not allowed to do any online gambling at all, whether its bitcoin or not.
You can use a VPN but keep in mind that if you send your withdraw to coinbase or xapo, they will blacklist your account because you are not allowed to send/receive transactions from any gambling btc addresses.
This is not true! Most US states allow online gambling, it is not allowed to send money to online gambling sites.
Despite of this, the operator has to state which countries are not allowed to use the service.