The whole US-only thing is a bit baffling. I can only assume that it has to do with licensing fees, HOWEVER, with Amazon apparently having done this without all of the "necessary" licensing in place, that shouldn't be an issue.
As one who can benefit from this, I have to say that I'm very excited by it. My issue is that, with over 200 Gigs of music on my home server, I can't quite fit everything on my cloud account, so I'll have to be selective with what I upload. I can say that the fact that anything I buy from Amazon is stored for free on the service will have me thinking twice before buying anywhere else.
It's interesting seeing Amazon turning more and more into a technology and content company and I still maintain that we will see an Amazon Android device before too long.
Curiouser and curiouser.