They definitely optimize for impulse purchases, which is the whole point of one click checkout and "free" prime delivery.
If you think about the process of purchasing as a series of decisions, each decision that one has to take is a chance to think "Do I really need this?" and bail out of the check out funnel. Say that delivery wasn't free, you could add add a $5 widget to your cart, then as part of checking out one has to think "is this $5 widget really worth $9 because of $4 shipping costs?"
Instead, it's possible to have this one decision shopping experience where the sole decision is "do I want this $5 widget" and if yes, click the button with no further input required.
