I would love to check this out, but there is a checkbox on the sign up page that states: "I certify that I am not a U.S. citizen, resident or entity (a U.S. Person) nor am I acting on behalf of a U.S. Person." which at first glance doesn't make a lot of sense, but basically means that you can't sign up if you are an American...
That's fine, but it should be much more clear before you even get to the sign up page, and not just put it as a checkbox that most people aren't likely to fully read.