But the whole idea of a free lunch started in saloons, where you'd buy a drink and get a "free" lunch. Except the lunch was so loaded with salt that it'd only make you more thirsty and you'd end up spending more money on drinks than you would have anyhow. Bars use the same trick with free peanuts, popcorn, whatever, to dry you up so you spend more money.
Makes you think you're getting something when the only thing you're getting is robbed and hammered. Not a bad metaphor, in the end.