William Hill is regulated by Gibraltar and powered by Playtech casino games. Both Gibraltar and Playtech do not accept citizens from the United States of America.
I sent an email to the William Hill support to get a clear explanation. Here is their answer :
This is Regine from Customer Service Team.
Due to changes in the law within the US online gaming is not permitted and we take active steps to prevent this.
William Hill Casino is licensed by the Government of Gibraltar and regulated by the Gibraltar Gambling Commissioner and as such fully comply with any requirements which they stipulate.
We would always suggest that you check in your current local country/territory as to whether or not it is legal for you to gamble there.
As part of the US gambling law, William Hill preferred to step back from the US Markets. However, some online casinos still continue to accept US citizens. Check our top 3 US Casinos.
Technically, you could play to William Hill if you signed up from abroad. For instance if you are in vacation in Mexico or in Thailand, you could play to William Hill while you drink your Mojito. However, you should not use a US address and an american credit card (from Bank of America for instance).
This is the same problem as "signing-up from abroad". You could use a VPN to play to William Hill, but you would need to have a real address outside of the US (with utility bill at your name for instance). If you have a sister abroad, for instance in the UK, it could work. Otherwise, in case you win a $1,000,000 jackpot, William Hill could block your funds..
We know that Bovada is currently accepting US citizens. It is powered by Top Game software and offers a nice welcome bonus.