I work for Amazon.com.
The reason for this is that ASINs (the 'B...' number referred to by the OP) are not technically global - they are tied to a given marketplace (.com, .ca, .co.uk etc). Products can vary from region to region, and may not even be sold in a particular marketplace.
In this case I suspect that the aforementioned Keysonic keyboard is not ...
This problem appears to be specific to products and not books. Consider:
but for a product, even an international one...
That site apparently uses the HTTP referer header which is part of an HTTP request initiated by clicking a link to redirect your request.
Not so in Safari 5, which seriously broke tabbed browsing, it's not even funny; therefore I couldn't reproduce the issue at first.
Using curl and manually setting the HTTP referer header:
$ curl --include http://www....