Why are British novels "translated" for American readers?

Reading Jim Crace's Harvest, I am confronted once again with the inexplicable practice of Americanizing British novels.
It is most disconcerting and jarring to be reading about quintessentially British things like "winter ales", "porridge", "tinkers" and "rooks", but juxtaposed with American spellings like "labor", "neighbor" and "traveler". I'm sure that's not how Jim Crace would have written it, so some American editor or publisher has taken it upon themselves to "translate" the story for an American readership.
Worse, that American translation is also foisted on us in Canada (despite the fact that Canadians typically use more British spellings than American). The book I am reading, with its American spellings, is published by Hamish Hamilton, the Canadian arm of the British publishing house, now part of Penguin Canada: it is not an American book that has found its way into a Toronto second-hand book store.
So, why do they do that? Why go to the trouble? Are American readers really not able to deal with a word like "labour", and make the link with the more familiar "labor"? Do they really need "car park" to be rendered as "parking lot", and "lavatory" as "bathroom"? It remains a mystery to me, and I have never seen a good justification for it.
It also make me wonder what else has been changed in the text I am reading. Other blogs have catalogued some of the enormities that have been enacted in the interests of American cultural imperialism. Harry Potter fans, in their usual obsessive way, have exhaustively documented changes made between the British and US editions in the Harry Potter Lexicon (remember the furore when "Philosopher's Stone" was changed to "Sorceror's Stone").
Surely, it's not too much to ask that we get to read to what the author actually wrote, and not what some publisher thinks we should read.