I doubt the consumer mass that actually matters to manufacturer's earnings understands RAM value and if the computer they are buying is RAM-upgradable or not.
They are going to buy the 800$, any of the two, complain when it inevitably "works slower" in a couple of years (if they are lucky), and buy a new 800$ once again then. I don't see the manufacturer's motivation to offer upgradable RAM.
They don't have $800 to buy another one so soon. So they take the one that "works slower" to some tech who knows the deal and tells them this machine sucks because you can't upgrade it, and now they think your brand is crap (because it is), curse you for the next however many years until they have the money and then buy the next one from someone else.
They are going to buy the 800$, any of the two, complain when it inevitably "works slower" in a couple of years (if they are lucky), and buy a new 800$ once again then. I don't see the manufacturer's motivation to offer upgradable RAM.