Makes sense to me. They've already spent the money putting the equipment in the car. Might as well take the upgrade whenever. Even from a second or third owner.
They are following the Tesla model. Perhaps it's not that much more expensive to give everyone the hardware than offer the upgrade later. If they are offering SSPro later then that must mean they all have, what is it, 21 speakers? Just use software to turn off speakers if the owner did not buy SSPro. I might be interested in getting DDPro if one day they have rock solid self driving.