Titles are put on books to sell books and for no other reason. Very, very few authors have the clout to have any say in book titles.
Editors think they know what kind of titles will sell in their country. They don’t, of course, but they think they do. If they think that a title isn’t good enough or right enough or doesn’t conform to some arcane secret of the publishing business, they will change it in a second.
This practice is changing somewhat because of the Internet making information international and instantaneous. But the number of books whose titles have been changed over the years must be in the tens of thousands. An amazingly high percentage of classic mysteries have different titles in the US and UK because of the way the two cultures saw book marketing differently. (Have you ever read Agatha Christie’s Ten Little Niggers, e.g.?)
This extends to all arts, including movies and records. Remember that the US versions of early Beatles albums actually left off songs because it was standard practice to have 14 on an LP in Britain and only 12 in America. Sgt. Pepper’s ended that, but the attitude never really went away.
And who is going to sue and why? It is illegal to sell US books in the UK and vice versa. All contracts have exclusively clauses built into them. So there’s no legal chance to get the books mixed up in the customers’ minds. I know you’re going to object and tell me that books are sold all the time across borders. Mail order is not the same as the bookstores in one country putting out unlicensed books from another territory.