It is news to me that this is not the way things work.
I mean really people, I hate to sound condescending, but... when you do a "buy one get one free" do you really think you got something for free?
Maybe one day we'll all get free healthcare and free schools and freeways too. Don't kid yourself, you're paying for them.
To reiterate the parent comment: No, but we are paying a non-trivial fee for Prime membership every year. Is it unreasonable to have thought that part of that fee would go to offsetting shipping fees to the original seller?
I found your post informative in that, no, in fact it does not. That was definitely news to me. But no need to be condescending about it.
Remember the classic EBay scam back in the old days? You'd sell some widget for $5, with $25 shipping. Commissions were calculated on the item cost and excluded shipping.
Also, Amazon is making rule/practice changes that are eroding the benefit of Prime. In 2010, they would send products to me that weren't in east cost warehouses via 2nd Day air. These days, they usually just deliver it late.
Building shipping into the cost like Prime however, might do the reverse and expose a seller to over-refund issues where they would lose more on a refund if it must include the total item price without shipping. (When shipping is "free" and built in to the item price.)
The real issue here is that amazon is accepting the $79 fee, but not passing any of it to their third party retailers, whose offers are still labelled as "prime" (when they are not).
In practice, though, it does appear to be more than that. The vast majority of the time, the difference between the total cost if I buy from someplace without free shipping, and the total cost if I buy from Amazon with free shipping, is very close to the amount on the "shipping" line item at the first place.
It is just an illusion, as you say, but they made the illusion so good that I cannot usually tell the difference from reality when I'm comparison shopping for something.
My wife is an Amazon seller and yes when she ships prime she must pay for shipping which would force her to lose money on each shipment so she builds the cost of shipping into the cost of the item. The cost of her taxes are in the item. The cost of her employees are in her item. The cost of software licenses are built into the item.
The only thing you get with FREE shipping is a receipt that doesn't include shipping as a line item. Someone has to pay the driver, buy the truck, and put fuel in it, pay for the insurance, etc, etc, etc. It can't possibly be free.