"You can't prepare something for shipping if you don't have it in hand."
Well maybe not, but what are you expecting, a shampoo of the item? You prepare the shipping products and paperwork, item is found or arrives, and they drop it in the box.
"Maybe "in stock" meaning "in the catalog" is acceptable to those regular customers who are in the know and accept it. But it's a blatant lie to everyone else"
It depends, if you get stuff is in 1 day, like you are located across from the QBP warehouse, if there were certain items where that was possible, it could be a hell of a lot easier to handle that, than a part that is on your inventory, and nobody can find it, or someone took a 5 finger staff discount on it.
They have listed 20 000 + items on Amazon, tough to keep all that organized between several web presences and a physical store.