The whole point of a shopping cart is for items that you are going to buy--not for things you may (or may not) buy tomorrow. It's a shopping cart. Not a bookmarking service. You don't go into a physical grocery store, put things in your shopping basket, leave, and expect them to be there when you walk in the next day.
That is your interpretation of shopping carts in web stores. Other people add things to their cart as they need to replace eg: items in the stationery cupboard, then checkout once a week. Some people use the cart as a form of Wishlist, or a picking list while evaluating similar products.
At the very least if a site doesn’t offer Wishlist, shipping list or other bookmarking facilities I would expect the shipping cart to give me a cookie that lasts at least three days to cover the weekend or the option to create an account to save that shopping list/cart to come back to later.
The metaphor is imperfect. It is a staging area for orders in practice, and people like to be able to use it that way. I often build up a shopping cart on several sites while trying to figure out where is the best place to buy from (especially in cases with a shipping is a large proportion of the overall price) and sometimes this takes me several days.
