It has to do with deceiving the customer. Yes, WE all know what the regular price should be, but when a company falsely states regularly prices (or never sells them at that) it CAN confuse some people, or influence someone to purchase something because they THINK it is on sale.
This is why the My Pillow company got knocked down on the BBB rating system. They had an offer of B1G1 pillow for free, just about all year, which got them in trouble. How can you offer something half off, but never sell it at regular price.
I'm not saying I agree with these kinds of lawsuits, though I don't necessarily agree with some of the business tactics either, but that is the thought process.