The memory in retina Macbooks is soldered to the logicboard. You cannot upgrade it. The amount it has when you purchase it, is the amount it will have forever.
To make the laptop slimmer, lighter and also to prevent users from screwing around with upgrading themselves.
A lot of people try to change parts themselves and end up breaking some other part. I actually prefer Apple to lock down everything so that people can't tinker with it and inadvertently screw up something else.