[ac29]

I'm pretty sure it is for nefarious reasons. For the last year or two (maybe more), Amazon has never failed to deliver my free shipping (non-prime) orders in 1-2 days from when they ship. Since this would be a disincentive to subscribe to Prime if they shipped it within a day or so of receiving the order, they just do nothing for 4 or 5 days so that it arrives in the 5-8 day quoted window.

Its clearly in their best interest to push people to subscribe to Prime to recover their costs, their order fulfillment is so streamlined that a standard free shipment and a prime shipment probably cost the same in 95% of cases.