I think it was to increase friction of resellers installing shady 3rd party ROMs right away. If I remember right, waiting period was implemented after drama with resellers loading ROMs with spyware a few years ago. Maybe thinking is big sellers won't hoard stock for multiple weeks. My understanding is vendors that offer unlock actually opens the phone and starts the unlock countdown first and repackages after. I don't think there's any official endorsement/support for it.