It's obviously to control server population, but it still sucks because it ruins one of the biggest draws for MMOs for many people: the social aspect of playing with their friends. We're in the very first hours of early access (AKA "launch" for the most part) and people are already forced to split up (and later reroll or pay to transfer if they want to play together) or not play at all.
And actually, at this very second, every single NA/EU world is locked.
