Authors sell their publishing rights both in terms of a geographic region and in terms of the media to be used for publication. So, for example, they'll sell 'first UK serial rights (print media)' or 'first US serial rights (electronic media'). If a US author has only sold the US serial rights their publisher can't directly offer the book in the UK, either by distributing it here or by making it available electronically. Of course you can still buy the physical book from a US retailer and then pay the import costs (as the publisher has no control over that) but you can't download books from the USA if the publisher doesn't hold the UK rights.