I don't know the exact details abut gambling, but I recall a recent set of laws or rulings by the D.C. Nanny Staters aimed at stopping innerweb gambling. I recall some Big Fish mucky muck got nailed in some U.S. airport on a bazillion charges. I don't know where it is within the "system". It may have been a show of force to all in that industry.The main law that dictates that online gambling in the US is illegal, is the 1961 Wire act, made before the days of the internet.
The law does not dictate where a server is or where an owner is from or anything like that.
It just disallows online gambling money transfers in the US.
However, another innerweb sin, online "pron" ;-), I'm pretty sure most if not nearly all the naughty servers are offshore, far away from the long arms of Nanny Sam. In that case, where the servers live is definitely germaine to legal enforcement.