I'm pretty sure that's a new thing for the UK cruisers. Since they like everything figured into the bill I'd say it's a good change for them. Surely you're not advocating for people to adjust their gratuities? Why do you think the change was made to have it included in the fare for them?
In answer to your question, I think people from another country cannot book with a US TA, except for Canadians, and they're almost like people from the US, except nicer. Now if they have a US address it might be different, not sure how close they cross reference with passports.