I think the shopping cart theory needs to be a bit more fleshed out.

I had assumed it meant people leaving carts all over the parking lot, not right at the exit of the store. The problem is that carts being all over the lot often block spaces or can roll into people's cars and damage them. If the cart is left right at the exit, those problems go away. It's also very quick and easy for employees to grab them there. If the customer isn't parked out in the lot, it wouldn't make sense for them to be expected to take the cart farther away from the store just so that an employee can bring it right back.