Pure speculation on my part...but I think maybe it is to allow passengers who book very late a chance to reserve a spot. Same thing with Chef's Table and Winemaker's Dinner. If you *could* pre-reserve as with excursions...all possible spots would be filled before these late bookers even had a chance. Yes...I know there will be those who take the attitude of "You snooze, you lose" as far as booking. I'm not saying that it should or shouldn't be done either way...just my opinion as to why they *might* be doing it this way.