I think Audible sell the narration of the book, not the FAST CDs.
Yes, Audible do use some sort of regional distinction in what is availble. I moved to the US several years ago, but was employed by a UK company and paid in the UK, so I kept a UK credit card going - it was on this that I had my Audible subscription (even though my address was US) and sure enough I had limits on what I could purchase. I was really surprised when I switched to a US credit card eventually and found out what a much larger selection I had available.
I imagine the restrictions have something to do with regional publishing agreements and rights.
As an aside - this impacts my listening to soccer on Radio 5 Live over the Internet. There are some games (including the FA Cup Final for example) where they won't broadcast outside of the UK and hence don't stream it across the net (or at least if your accessing I guess from an overseas IP). It's all very frustrating.