Well I hope you consider changing this and soon allowing USA residents to use your site.
That decision might not be theirs to make. Although I don't see any proof of a betting license on the site, they could be applying those standard Curacao betting terms which restrict countries like the United States, France, the UK, and some others.
When we are on the topic of restricted countries. @BetJA.com: I couldn't find a list of countries whose players aren't allowed to play on your site. Can you add that to the T&S or state it here for us to know?