#10 It's still a "British publishers' prize". To be eligible, you have to be published in the UK by a UK publisher. Books published by an Irish publisher, for instance, aren't eligible, ditto American novels that don't have their own UK publisher.
I doubt there were US publisher pressures to open up the prize. There aren't so many UK publishers who are owned by US publishers, and just because a book is published by, say, HarperCollins in the US, doesn't automatically mean it's published by HarperCollins in the UK. So I don't think US publishers have a vested interest in the Booker. Of this year's six shortlisted books, three were from British independent publishers. The other three, all Penguin Random House books in the UK, were published by non-Penguin Random House publishers in the US.
Which is not to say I don't also think it's a shame that it was opened up to American authors -- American fiction is hardly overlooked in the UK, and they have plenty of prizes to recognise their own talent already. Having said that, this current winner -- the first US winner since the change in the rules -- wasn't a 'big title' from the US, so it's going to get some welcome coverage after winning the Booker.