I guess the court case proves that Delta can complain.

The obvious math is -- there is always a hidden city fare created whenever there is a sale to anywhere, since almost certainly, you have to go through a hub. So, if you fly from SLC to CVG, you can pick the lowest fare from SLC to anywhere, and route yourself through the hub.

If the airline can't protect against that, we're not going to see many sales, are we?

The sale price is not the "real" price. The sale price is attached to certain conditions. If these conditions aren't met, they we're not eligible for the sale price.