I think there's still the possibility (at least I haven't seen it shot down) that some of the places will have a guest-accessible second floor, like WWW and maybe even the Leaky Cauldron, plus HTF did say that there will be stairs inside Flourish and Blotts. If that is the case, they would want the view out of any windows to be complete.
Just by my own logic, it seems with how tall these buildings are, why wouldn't they include more than one floor?