They decide what the price is. If that price is so high that they can give away an extra product while still turning a profit (which they are, or they wouldn't offer the deal to begin with), then the discounted price is the only price even close to fair. The regular price is them overcharging you. This does not make the discount good.
Now, imagine how much they're overcharging you if they give you a "buy 1, get 13 free" offer. That's what this is.