Why do Americans spell words like "theatre" with "er" instead of "re" at the end?

@Guardian I thought the French blamed Britain, not America, for its imported English terms, though they import American as well as British ones.

After all, France and Britain are neighbours with a lot of business, social and leisure travel between the two countries.

I don't know which country coined "week-end", but have noticed in radio interviews, instruction-leaflets supplied with imported goods, etc., that many people around the world speak American, not British, English - even when their countries officially despise the USA. Much of that is due to the USA flooding the world with its films, TV shows and pop music.

Australia, a former UK colony and now a member of the Commonwealth, seems to think herself an odd mix of EU and USA: speaking a possibly-American-influenced version of English, using metric units and calling its currency "dollars". Though of course as the USA is now the only nation not using metric units in general life, Australia is not really copying anyone in particular there.

@ArishMell As a child, I witnessed Canadians using more British English, but as I have grown, Canadians have now switched to more American English. We of course, Canadianize our English with our own ideas, including Franglais, a blend of Québec French and Canadian English!

@Guardian I can understand that, with being the USA's next-door neighbours. Has there been any traffic the other way, with the USA importing some British-isms?