My guess is they're barely breaking even or taking a loss with the nuggets. But, they're planning on people buying fries, or a drink, or whatever else. Im sure yge vast majority of customers arent buying just the nuggets, so they more than make up for it with your otherpurchases.
We do a $5 burger night at one of the restaurants I work at. Legit 8 ounce burger, any way you want it (bacon, shrooms, carmelized onion, various cheeses, sauces, etc... as much or as little as you like) with fries. But, you have to buy a drink (soft drink or alcoholic). We barely break even on the burgers, but do well that night just by volume. We average about 90 burgers a night on a Wednesday.