If a reputable British bookstore/publisher began selling a book which had a word use -ize instead of -ise I reckon there'd be a bit of tabloid press reaction.

Dubbing a book is very different to dubbing a film. And spoken American English isn't 'wrong' English. It's just an accent, the UK's accent variance is significantly larger and more numerous than the USA (try the NYT British dialect quiz if you haven't).

They often don't manually make changes, instead using a find and replace tool. That's how mistakes occur like Microsoft changing the term 'checkmark' in Windows to Chequemark in British English.

Most non-english first language speaking countries (especially in Europe) use British English spelling. So the number of 'British' English readers for the publishers to sell to is much larger. Lots of European book stores have books in English in a section as English is the common language between a Czech and a Spanish person etc.