Also why would Apple use the word bag for what is clearly a cart? I don’t know anyone who puts unpaid for merchandise in a bag unless they don’t plan on paying for it.
Probably some user study they performed told them "Bag" works better because, as you say, your put stuff in your shopping bag
after you've purchased it but something in your cart hasn't been committed to just yet. So you can avoid the decision baggage (pun not intended) associated with something in your "Cart" and therefore remove another symbol preventing people from backing out of the purchase. Also "Cart" reminds people of a rickety shopping cart at the grocery store. "Bag" reminds people of their nice leather handbag.
Maybe I'm not getting the details right but no doubt a company like Apple ponders every single variable in the shopping experience. Much like setting the MacBook displays at a certain angle so you have to touch them or having the products on display for you to pick up and feel like it's already yours -- everything is designed to make the sale happen.
