People still pay it, because most people probably don't know the RAM is replaceable, don't want to be bothered with doing it themselves, don't want to accidentally damage their new computer, don't realize the upgrade prices are high, or other reasons.
Apple has always charged high premiums for upgrades... premium upgrades aren't a strategy they suddenly invented when they started soldering components down.
Apple has reasons for soldering down components, reasons which I don't really agree with, but I don't think they're all that worried about a small percentage of people avoiding their upgrade prices.