I was thinking the exact same thing.
The only rationale that I can think of is that they want to put up a SDCC section all at once and while the Carbon Freeze chamber may have sold out at the convention, other exclusives did not - so it's those pieces that need to be worked out.
I've said it before and I'll say it again - I don't understand why they just couldn't make it so you could pre-order this thing, pay one price for pick-up and then pay another price for shipped-to-your-home. I'm sure there are even people who go to the convention who would take the ship-to-my-home option just so they didn't have to deal with the big thing at the show.