Actually, I think it's just designed more with American users in mind - typically U.S. paper formats are 'squarer' than those use in the UK and Europe, which has pretty much standardised on 2:3 format 'A' paper sizes. I think 4:3 will work quite well when using the on-screen keyboard in landscape mode, as the keyboard covers less of the available screen, albeit at the irritation of folks who buy an iPad to watch movies
