It's an old economic chestnut that there's no free lunch. Anything that's worth something generally costs something - even if it's just the effort that it takes to pick an apple off a tree. And one of the most common deceptive practices in advertising is the idea that you can buy something and then get something else free. The word "free" usually doesn't belong.
A current exemplar of this unfortunate phenomenon is the fast-food chain Sonic, whose television ads are offering a "free" medium order of tater tots with the purchase of a double cheeseburger. The fact is, anyone who takes advantage of this deal is still paying one price for two items; Sonic simply decided to charge a lower price for this combination during the promotional period. In principle, that's fine - advertising based on price is one of the forms of publicity that actually provides useful information. But the word "free" is unnecessarily deceptive. Just try ordering the "free" tots on their own, and see how far you get.
(By the way, does anyone actually prefer tater tots to fries, even if they are delivered on roller skates?)