Earlier quoted context omitted.
> Every unit Amazon ships probably arrives in a "Prime" amount of time anyway, giving away expedited shipping for free Just to add a note that this isn't always true. Sometimes they withhold shipping even though they have the item in stock at the nearby distribution center.
This is definitely true. Back in the pre-Prime days (ie, 5+ years ago) Amazon would send your package as soon as they possibly could, and even with the super saver shipping you'd still often get your order less than the minimum 5 days. Now I'll order something on Amazon (not a Prime member) and they'll sit on the package for a week and then overnight it to me. I'm not a fan. Paying more for faster shipping didn't use…
I don't remember exactly what the other reason was, but it involved a one time money savings which was > the cost of a year of Prime at the time.