I can see the argument with supermarkets, since clearly you have to go there, and find no stock, and end up buying something else again. I guess that it's not quite as true about an online retailer, where the effort involved in going to another store is substantially smaller.
Of course, if the same price is meant to be available in-store, then clearly it could be argued that this is an inducement to go to the store, and in effect it's an advertisement (although not quite so blatant).
As has been said, they don't seem to have bothered to remove the item, and have actually dropped it in price (whilst remaining out of stock...) which does seem either spectacularly stupid, or suspicious.