It is an unintended consequence and I believe a dev posted that is was a design mistake that they wish they could do over.
But the basic idea was that the 'perk' was tied to the RETAIL code (which you do not enter until the 2cd) instead of the Open Beta/Pre Order code. It was feared that people could order the preorder from 5 different places, getting 5 different preorder codes. Add them all to your account to get all the goodies. but then only buy ONE retail copy and cancel the other pre-orders.
They admit they could have implemented it better.