You should brush up on your reading comprehension because the lunch was literally free, people did not pay for lunch or, if we consider your view, far less than what the lunch is worth. The point is that you could pay only for the cost of beer and the lunch comes free. If you bothered to read the Wikipedia article you would see that "These establishments included a "free" lunch, which varied from rudimentary to quite elaborate, with the purchase of at least one drink. These free lunches were typically worth far more than the price of a single drink.[1] The saloon-keeper relied on the expectation that most customers would buy more than one drink, and that the practice would build patronage for other times of day."

It was a marketing strategy for bars, sell at a loss and gain a profit another way. The point remains however that they would literally give you a lunch for free, something that I did not know actually happened.