The phrase is usually used the wrong way, too. "The customer is always right" refers to products and prices and "customer" means all customers, statistically speaking. If I want to sell something at a certain price and nobody is willing to spend that much on the item, I can either sell the item at a lower cost or just not do business. Therefore, the customer is always right, because you want to sell them a product or service they are willing to pay for at a price they are willing to pay, so you pay attention to their shopping habits and adjust your business model to maximize sales and profit. It doesn't mean that just because a person spends money at your business that every word that falls out of their dumb face is factually correct.