When a business structures a deal of this nature they are doing so with their bottom line in mind, and should know that whatever money they recoup from selling the deal will at least cover their costs.
The main idea behind running these deals is to get new business in your door while minimizing the cost of advertising and promotion. It is up to the business how they choose to retain this new growth, and most businesses employ great customer service and or a slew of other great products at full price.
As Fred2baro said, this the businesses chance to engage with a customer you would of otherwise had no access to. That in itself seems worth offering something at cost one time to that new customer.
In the case of a restaurant, this is no different then the traditional offering of a "2 for 1" or a "50% off" deal through traditional media. In both of those situations the restaurant is showing their "real" price, as you said, to the would be customer, but with no guarantee of use. This is where the daily deal truly shines, for in order to take advantage of the deal, the would be customer has to put up money, i.e. purchase the deal. Talk about a brilliant incentive to get someone into a business, they are paying the business for the chance to win them as a new customer. As a business owner, I don't understand how this could be a negative.