It's legal, though less than ethical. I think it's a relative of the old "bait and switch" trick used by car dealers, where the low advertised price that gets you in the door is for the stripped-down model they expect nobody to buy.
Another example: the triple bacon cheeseburger. With that on the menu, you can chow down on a gut-busting double burger and still feel virtuous, because you didn't buy the "big size".