I think that considering this book got bazillions of kids to start reading, although of course the publishers didn't know that at the time, it would be fairly stupid and frankly quite sinful if said kids ran around not only wearing witch hats but asking the local bobby where the nearest lorry stop was and to please not to ticket them for using the zebra crossing whilst the lights were against them, not to mention failing all their exams because they used British spellings like "colour" and "grey" and "aluminium." It's fine if you learn American English first and then read a British book but not okay if you learn British grammar without even realizing it. Besides all that, you'd need a bloody glossary at the back of the book, not because of stupidity, but because Americans simply don't KNOW what a lift is. So why bother?
Anyway, do you complain that the Bible was translated to English? Harry Potter ain't Shakespeare, it's just written in another langauge. One that invented the letters H and R but fails to pronounce them all the time. And sometimes T. Now tell me, what's the point of that? 'Arry Paw'uh indeed.