On a stack of books, though, it does make a difference. Books below the top book do not have a readable cover.
All English-language books I've ever seen beside foreign-language books (so that I could notice the distinction) read from top to bottom. It's not just the US.
I've only seen Top-To-Bottom-Spines on US-Books so far. Also agree with the point, that if a book is lying face-up there's no need for readable spine-text. If it's face-down, there is.