They do accept USA customers, but when you are wagering, you just can't be physically in the USA at the time which I assume they can figure out via IP addresses. I realize that discludes alot of people, but if you are a US citizen living out of the country, you can still sign up with your US address and make deposits and wagers. And I thought I heard about some legislation in the UK that will soon permit wagering from within the USA.
