Speculation, but maybe they wanted to get caught? What better way to cause hell in the Middle East than provoking NATO into something, and there seem to be a rather deliberate set of pointers to the Arabic world, don't there? For example, why leave Arabic flight manuals in a hire car at Boston Airport? The hijackers now appear to have been of Middle Eastern origin, so it cannot be an attempt at misdirection. Why do it if you are trying to be covert? It's hardly going to be last minute revision...?
Like I said, speculation. Who knows?