Although a lot of pathnames match across US-UK, I don't think they've fully harmonised all the IDs or have any efficient way to tell which ones actually match. So apparently they're just playing safe by redirecting to homepage.
I'm not surprised as even those geo-targeted "Continue shopping" messages are fairly recent. I once wrote a Teleporter script to switch between Amazons (http://softwareas.com/domain-teleporter-greasemonkey-script) and if they're redirecting to homepage, it's still useful!
