While the official Pirates site doesn't have a specific place that says "US Customers Only", the specific intention of the current site was simply that. As other users have mentioned, it is possible to pay by using a credit card to make a direct payment to Disney from the Pirates Online site. It is simply a choice of Disney to run a separate server of the game for each country it decides to add, and has been this way with all of their online games so far. The only exception to this is Toontown US and Toontown UK. They share the same game server, but the login and payment servers are different.
Disney has also been working on a UK side of the
Pirates site. The site has not been updated in quite some time though and no mention of it has been made since Disney first announced it.