Soldered RAM is part of Microsoft's specification for Connected/Modern Standby, supposedly because it prevents some classes of cold reboot attack on disk encryption. The official line from the people who work on the standards (both at Intel and Microsoft) is "buy a laptop with the amount of RAM you need." I hate it too, but it's not like they're pointlessly screwing their customers.
Which conveniently leads to earlier upgrades (everybody but the consumer and the environment wins!), and more profit to the OEMs in the form of RAM upgrades. Maybe I'm just too cynical.
