As the West Coast gateway out of America, it looks like the store has a good stock of books for across the Pacific; China and Australia are the books stacked vertically, and these are also the thicker volumes Lonely Planet prints. Since LAX doesn't have direct flights to Buenos Aires, and the Argentina LP is hardly as thick as China or Australia, I would suppose a Lonely Planet Argentina would have been arranged with the spine facing out.
In that arrangement, I think you could squeeze at the very most one extra book in the middle cell, where Argentina would probably be. But one book missing doesn't mean very much at an airport as busy as LAX.
Going by the rest of the display, only up to three books of the same title would be put on the shelf at one time; if a huge group of the same title is sold from the shelves, the staff would most definitely replenish the shelves with the title (they have massive stocks) instead of shifting up every cell to fill the shelves end-to-end as we see here.
In that scenario, the presence, rather than the absence, of a book would be more indicative of teams all jumping on a book.
I'm not 100% sure on the conclusivity of the photo... We should go back to work some form of torture on the clerk!
(LP's "Buenos Aires Encounter" has been left untouched on the middle bracket, but really, who buys these mini-guides anyway?)