>Why does that logic not go the other way, too?

Because the "sticker" price is prominently advertised, so advertising a lower price than nickel-and-diming up leaves a sour taste in people's mouths rather than advertising a higher price then offering a discount, even if the final prices are the same. Of course, everyone shops for flights on comparison websites, so their only metric is that sticker price, and airlines have to engage in this sort of stuff to be competitive.