How do they restrict it then? just a mass dns ban on sites that offer gambling?
But ther is nothing stopping you using fake information, theirs no postal confirmation or anything like that before you start playing.
My understanding, is that if the company allows a US user to gamble on their website illegally, then the US Govt will come shut them down... so if the gambling site wants to remain open, they must block the US IP addresses, or risk getting shutdown by the overreaching US Govt.
I'm not saying it's right, I'm just saying I think that's what happens...
So the end result is that most internationally gambling sites avoid US users. The ones based in the US don't exist because it's illegal.. hence, it's difficult.
If you're caught or suspected of using proxy/VPN/tor, they try their best to ban you to not get shut down.