Hm -- can you drop me a note? (patrick@stripe.com.) I'd like to dig into why. It's still invite-only but you shouldn't have been turned down because you're in the US.
I haven't done it yet but I just found that Gust offers a similar service to Atlas with a notable difference being that Atlas opens the back account for you.
But, based on Patrick's comment, maybe Atlas is now open for everybody.