Suppose I were to advertise a car for $18000 where the best deal at an equivalent dealer is $19900. When you arrive at my store, you find I only sell it for $20000, and that I never sold any at $18000. The prices on each of these cars is clearly displayed, and at the time you are looking at the car, it is clearly priced at $20000.

Say people come to the store after having seen the advertisement, but when they arrive they decide that after coming here all the way, they don't care to save $100 and they buy the car.

It is conceivable (even probable) here that if I had honestly advertised it as $20000, few would have come. Therefore, making this change would increase my conversion rates on my advertisements. The argument stated by you would mean that my behaviour is defensible, and that I should advertise prices I do not intend to deliver, allowing advertisement conversion rates to be my guide.

However, that's a classic bait-and-switch scam, a kind that is illegal in the US, and I doubt you'd find anyone who'd support it.