You must not live in the United States... Where it is difficult to gamble online with fiat.
How do they restrict it then? just a mass dns ban on sites that offer gambling?
Yeah, how easy it is to gamble online will depend on the country you are in. Some places it's banned completely whereas others it is heavily regulated. It is quite easy to gamble in the UK but you have to give them your address and a lot of personal details which you don't with bitcoin at the moment (though this will likely change with regulations etc (especially if they want to operate in certain countries)), so in that respect gambling with bitcoin makes it much easier currently.
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.