That's because it is. The Atomic Avenue store simply isn't set up for pre-orders. It is set up for customers to purchase existing inventory from individual users of the ComicBase program.
I doubt that Human Computing is looking at doing preorders themselves, especially since that space has several large competitors that offer substantial discounts (DCBS, mycomicshop, and Midtown come to mind). Pete can correct me if I am wrong, but that is my hunch.
I don't know if it would be feasible for Atomic Avenue to be updated to allow pre-orders from specific sellers on the site. Pete would have to weigh in on whether that programming would be possible/practical and, if so, whether the AA admin would be willing and interested in overseeing those types of customer/seller transactions.